Hi Chris, most excellent work...one thing I noticed...DEW controllers using RCA connectors.
You have designed the system as a low side switch I'm not sure if you are controlling the TC4427 outputs with PWM (I suspect you are) or just switching the MOSFETs on/off.
If you are PWM controlling the FETs then...if applied power driving the mosfet is low,
the outside shield of the RCA can approach +12V.
Touch this against any part of the mount (earth) then there is a short cct to earth.
Somewhere back in the dark ages someone used RCAs for DEW strap connectors and this became a pseudo standard...a poor standard at best, especially if the RCA plug is a metal one, like some used by Kendrick. Although I have seen some of these covered with heatshrink.
The best fix.... use a high side switch MOSFET (wire the RCA shield to grnd, thus will always be grnd) like AUIR3315 (or called IR3315), 3~30A programmable shutdown (via resistor) high side switch...even has rev polarity protection...no fuse needed!
I have tried them...they work a treat and impossible to destroy once I shutdown set. Specifically they are designed for Automotive work, thus ideal for +12V systems. There is a range of these FETS, IR3313 ~IR3317 that have varying properties specifically the IR3315 is best (~$3.9 in to220 format) due to the low on rds and I shtdwn can be as little as 3A. If a dew strap is using more than 3A...then well yikes!
Sorry for the long read...
Brendan