led flashing to music

I did a small project today using a TIP31, 300ohm resistor, a 3.2v led, 9v battery and some computer speakers for input I want to do this same project in my car but on a larger scale, I have LED strips inside the truck where feet go, 5 of them. They all operate on a 12V source and are all connected together into 1 end wire with POS and NEG. Can i use the same setup as my small project and use the input from my doorspeaker, and remove the 9v battery and supply it with 12v from the car battery?? Thanks

You don't even need the transistor unless you are using that as a filter. Just place a string of LEDs across the speaker. How many? P (speaker) = V^2/R where R = speaker impedance. Hence if 100W into 4R (Ohm) 100 = VxV/4 => VxV = 400 => V = 20V max. Hence up to 10 2V LEDs or 5 4V LEDs, else use dropping resistors.

The High mount LED stop lights GM used to use on their full size SUVs and mini vans became a "HOT" commodity around me. For a while you would see more cars with them missing then not. Turns out they made really good BASS LIGHTS when you just hook them up to your speakers. Anyhow agreeing with oldspark if you have them setup for a 12v strip you should be fine powering them off your speakers directly. Unless you want a "constant output" regardless of volume then it will take a bit more work.