First roadster build

Chewy this has been a great thread. Finally got through it all, so many good things that I've taken into account as I plan my build. I entered for the LCS raffle the other day but didn't get the call last night so I'll continue as planned haha. I'll keep following and hopefully be able to add another Ohio roadster to the list

Looks sharp and clean.

How are you liking the HiPo?

If you swap it out to a 20A you (more than likely) need to run a larger power feeder wire from the 20A fuse to your heater switches. After the switches you should have inline 10A fuses, one for each heater.

It's possible a wire got pinched and is shorting to ground now. With the heater switches off, put a new fuse in. If the fuse blows immediately then the problem is between the fuse and heater switches. I'd then turn one heater on at a time and see if the fuse blows. If it blows nearly immediately then it's a dead short somewhere. If it's the heaters drawing too much current the fuse may take a couple seconds before it blows.

I would plan for the failure to happen again. The heaters may be pulling hypothetically 9.5A or even closer to 10A. That fuse is going to get warm since it's operating continuously at it's max rating. Something as simple as another large load turning on could cause the voltage to dip for a short period. This will cause the current the heaters draw to increase, this possibly putting it over the 10A threshold.
Unless you have an ammeter you can use to see what they're actually pulling and verify it's not close to 10A.
Do you know what the heater resistance is?