Electricial issue question trouble shooting

[FAB559]

i have my 65 c10 been the owner for about 6 months drives like a champ .i just have one issue that i don't know how to trouble shoot in order to fix so here goes hope this makes sense .

i have a red alarm like light to the right mounted to the dash
the previous owner said it was a precaution for when truck over heats
it won't start until light resets ,( temp gauges work fine)
when light comes on sometimes it won't start for like 2-3 hrs which is crazy sometimes 5 min ,and thats after sometimes driving for 3 hrs or around the block after a wash i have tried following the wires under the dash but its a big web of wires dash would need to come out which i will do as a last resort because its my DD and can't afford to have it parked all week truck runs a SBC 350 and when light is on i still have power to radio lights come on just does nothing no sound as if i turned a ignition with no engine or power to it

I Hope its something simple i can access in the engine bay

where do i start ? thanks for help in advance