Any heat issues and connector melting would be the result of a dirty connection, bad crimp, corrosion etc. Workmanship issues.
(assuming the wiring and fuse block is sized correctly).
That't right. The only reason Warm n Save might want you to run their harness directly to the battery might be because of dodgy wiring to the fuseblock. Since the heated clothing has a high electrical draw, you must make sure the feeder from the battery to the fuse block is heavy enough (i.e. the wire gauge is large enough) for all the loads, the fuse block itself is rated for the required current, and all terminations are clean and tight. I would not use any dielectric grease on these terminations.
If Soontobeone had problems with his unit, he must have had either too small a feeder to the block, or a fuseblock that could not handle the load. Overheating and melting of the wire's insulation indicates the fuse block was overheating - since the wires did not overheat when connected to the battery. Look at it this way - the wiring in your house has (probably) a 100 or 200 amp service drop from the pole. The breaker panel/fusebox bus has the capacity to handle this much current and the individual circuit breakers then protect the wiring attached to them. Overheating and melting of the insulation on wires attached to the breaker indicates too high a load with heat being produced by the breaker. If the load is so great that the wire overheats in the middle of a span and melts the insulation then the load is too great and the breaker is oversized or not doing its job. Note that there is higher resistance at wire termination points - higher than the wire itself. So a loose or dirty connection will produce more heat than a clean properly tightened one.
The loads for household appliances are much greater than for any load on a bike - and how often do you see melting insulation on the wires in your house (breaker panel)? It's all about correct wire sizing, properly made connections, and equipment that can handle the load.
