We don't really have a section for gadgets/effects and so on, so I guess this would be at least one of the sensible choices for where to put this..

Ok, so assuming you have an electric violin with a jack that works for plugging a headphone into.. You can get one of these thingies..

qid=1427485484&sr=8-1&keywords=maxell+fm+transmitter

Then you can play wireless and not be stumbling over a cable or having the cable dragging down one side of your instrument and throwing the balance way off. 

If you just want to hear yourself, any stereo or table radio will do.  Car radios pick up the best, but only if you are outside the car.  That can work for outdoor jam sessions though.

Range isn't as far as a pro wireless system, maybe 20-30 ft.  Sound quality is pretty nice.

Now, if you want to go "whole hog", you can take any old stereo that has an "aux out" or "line out" and run a cable from that to pedals and an amp.

But for just a quick wireless for your EV that will fit in the accessory compartment and where there will be a radio or stereo you can play through in at least most places you might visit or go to jam.

It *is* battery operated.  But I get about 28 hrs to a set of 3 AAA alkalines. 

Note to Bill:  The UK allows these type devices to have a little more power than the US does, so you'd get a bit better range.  Same for members in Canada.  But you may have to try a few brands to find one that you like the sound of and that will work well for you.  For US option, I've used my Maxell one a fair bit, so I can say it's fairly tough and sounds decent.