Step 1: Picking a Mosfet
there are two ways to do this. Use P channel mosfets to switch the power lead or N channel mosfets to switch the ground lead.
I made an 8 channel one for a traffic advisor clone for my truck. I got a bunch of 3rd brake lights from a surplus house for $2 each. I mounted them to a piece of kindorf and used a couple of muffler clamps to hold it to the rollbar. they all share a common ground lead so P-channel mosfets were used. that requires a 2n2222 transistor to pull the gate down. if you tried to hook the gate to the micro directly the mosfets would partially turn on and heat up. the trick is to always run them fully on or off. there will be very little heating.
Step 2: Single Light Controller
Step 3: Double Light Controller
Step 4: Triple Light Controller
this one is great for those RGB light strips. You can replace the pic micro with an arduino to make it easier to experiment with. just remember to use digital pins d9 d10 and d11 to drive the mosfets. Many of those strips are common Anode style so you need to use the schematic with N-Channel Mosfets….
For more detail: Generic Automotive Light Effect PIC/Arduino Modules