Easy, cheap, WORKS!
The easiest way I could come up with to do this and it works well, cost about 10 bucks. Buy a condensor microphone from an electronics store for about $10 with a 3.5mm audio plug. Remove whatever plastic crap the condensor mike is housed in. Glue the mike to a piece of scrap steel with 5 min epoxy 1/4 inch (6mm) thick with a 10mm hole in it. You are basically done now, bolt it to the block through the hole, run the wire inside the car and plug it into your stereos 3.5mm line in jack ( most modern car stereos have one). Use your stereos bass and treble to tune out unwanted frequencys and volume to adjust the.......ah, volume. Easy as that, hope it helps.