I'm looking to drop a sub in my 86 extended cab possibly one 15? Haven't messed with audio to much does anyone know of any boxes that would fit behind the passenger seat that can house a 15 and what amp wattage won't put a strain on my alternator?

 

Q-logic is one of the few companies I know of that build drop in fit boxes. However I don't think they build anything for this body style. I think only boxes that fit the toolbox panel in later 4runners.

So you will have to build or buy a prefabbed wood box.

Amps are only as good as your alternator. And you are the only one that knows it's condition. A good electrical system can handle 200+ watt amps without any additional help, crappy charging systems will dim the lights with a 50W amp.

Use big dedicated cables from the battery to the amp. Don't steal power off another circuit.