In my experience CFM and Pratt & Witney make the best electrical harnesses. I don't think I've ever had to repair one. They are solidly built and are almost completely enclosed in heatshrink with bonded boots.
Rolls Royce on the other hand their wiring is pretty poor in comparison. Numerous chafing wires, badly routed harnesses and little protection in the way of heatshrink and boots.
Having said that, if you were to get a wiring fault on a CFM or P&W, a repair in-situ is almost impossible, you would have to replace that section of harness. On a Roller you can easily do repairs. EGT harnesses are tricky to repair due to the fact that it has to be a continuous loop. In line splice repairs are not allowed.
A heat gun can be used on wing provided it is not a naked flame type. I have used one in the past that used a nitrogen supply and a halogen heat source that was approved for in-situ use.