Two 12v fan motors will pull quite a bit of power, our school bus fleet at work runs 20 amp fuses for each two motor heater using 14 gauge wire. The wires are quite long so the big wire is needed. They use every bit of the 20 amp fuse, just a little corrosion at the connection will knock out a fuse or trip the circuit breakers. These are likely bigger motors than yours.
If you will be running both motors at the same time, I would use a relay with a heavy wire off the amp gauge as a relay feed, then use the ignition source for the switch to turn on the relay.
Powering off the light switch could leave you in the dark if running lights, defrost, and heat all at once. The light switch should have a built in circuit breaker designed for the lighting load, adding in two motors could put the amp load too high for the breaker.
