Why does Smoke came from under passenger seat and smells of burning?

I can't think of anything that would be placed under a passengers seat that would do that, so look for a power wire to a remote amplifier installed after market, a possible power seat motor malfunction or seat heater problem.

Problem is that any of these would be fused and would blow the fuse long before smoking, so it would have to be incorrectly installed without a fuse.

Or someone dropped a burning cigarette under the seat.