Radio Shack sells a small battery operated amplified speaker which is ideal for use as either a portable or permanently installed remote speaker for a radar detector. It is their Mini Amplifier - Speaker catalog number 277-1008C.
To ideally use this speaker with a radar detector, you will want to make a few simple modifications to it. The first thing I did was to remove the rear cover, peel off the sticker, and then stick the sticker to the inside of the rear cover so that the outside of the rear cover is free for adding strips of Velcro for mounting purposes:
Next, I removed capacitor C5 and resistor R5 since these two items tell the boost the IC amplifier chip's gain by 10X which is way too much. You can use a pair of nippy cutters to remove these two items although I chose to desolder them:
Next, we want to increase this remote speaker's input impedance and balance the input relative to ground so that the remote speaker's volume knob will provide a nice range of volume adjustment when used with your radar detector. Additionally, what we are about to do will set the remote speaker's minimum input impedance to a much safer 22K ohms in order to preclude any possibility of damaging any radar detector which this remote speaker is plugged into. So first we must remove the tiny screws securing the circuit board so that we can then flip the circuit board over to do these simple modifications. Following is a photo of how to do this, and after the photo I will describe everything shown in the photo and why it was done. NOTE: The 220K resistor shown in the following photo is what I originally used and which worked to provide a full adjustment volume range with my Bel STi. I have since changed this 220K resistor to a 100K resistor in order to provide a full volume adjustment range with my Escort 9500i and my V995. So, as it turns out, a 100K resistor is most likely the ideal resistor for you to use.
In the above photo I show where the capacitor C5 and resistor R5 were desoldered and removed. As mentioned, you can simply instead use nippy cutters to snip out these components since they are not needed. Next, note where I cut the trace going from the center connection of the audio input jack to one of the contacts on the volume control knob. I cut this trace using a sharp box cutter knife, and then checked that this trace was truly severed by using a VOM meter. If you don't have a VOM meter, then simply cut this trace in two or three places to guarantee that you have cut this trace and that it is now a dead trace. Now we will replace this cut trace with a 100K resistor rather than the 220K resistor which I have shown, since a 100K resistor turns out to be much more ideal. The reason for cutting this trace and then adding the 100K resistor in place of this trace is because, in the original design, the volume control knob when set to maximum volume allows the input impedance to drop to a very low 1.5K ohms. This is unacceptable in my opinion since in the audio industry, line level inputs generally must have an impedance of at least 20K ohms.
At this point, we have the audio signal from the audio input jack passing through a very high resistance 100K ohm resistor. That is safe and fine and dandy. Yet the voltage coming from the radar detector's speaker jack is still too high. To resolve this issue, we now install a 22k ohm resistor across the audio input jack's center pin connection and the audio jack's ground connection as shown in order to create a simple voltage divider circuit consisting of the 100K resistor and the 22K resistor. The result is that the voltage level from the radar detector's remote speaker jack has now been reduced to a level which is appropriate for the amplifier IC within the Radio Shack amplified speaker.
Finally, after putting the circuit board back in place, be sure that the Radio Shack speaker's wires are away from the case screw socket as shown. You don't want either of these wires accidentally getting pinched when you reinstall the bottom case cover:
-- In order to tweak the Radio Shack speaker's volume range to suit your particular radar detector's speaker jack output, you can play around with using resistors between roughly a 50K to 500K resistor in place of the 220K resistor which I originally used and subsequently replaced with the 100K resistor as described above. No matter what, don't change the 22K resistor which was added to the circuit.
-- A 9V battery in the Radio Shack speaker will last for approximately 40 to 50 hours of operation.
-- The JRC386D amplifier chip (that darned chip goes back well over 20 years, but China still produces it!) used in the Radio Shack speaker is designed for a nominal voltage supply of 4 to 12 volts. Its absolute maximum voltage supply rating is 15 volts. Thus it should be completely safe to use a custom power cord to permanently power the Radio Shack speaker from your car's electrical system since most car alternators output roughly around 13 volts. Note that the Radio Shack remote speaker's external power supply jack is stupidly designed with a tip negative connection rather than a tip positive connection. Keep this in mind when permanently wiring up a plug to power the Radio Shack remote speaker. I would also very strongly suggest installing a 2A fast acting fuse on the power input to the Radio Shack remote speaker should you decide to power it via your car's electrical system.