Ive got some corrosion in my wiring leading to the glow plugs in my vehicle and I'm looking for some advice on how to tackle it long term. 

A couple of years ago I noticed corrosion on the main wiring loom where it connects to the car glow plug harness. It actually ended up melting the plastic where the harness connects to the wiring loom. As a result I purchased a secondhand glow plug harness because mine was basically toast. I've since learnt that is a common problem for my vehicle model (2009 Jaguar X Type 2.2l Diesel). So, I would love to figure out a more long term solution.

In the image, the red cable is the glow plug harness. That is actually in good condition. However, the main wiring loom has seen better days. It's obviously corroded and the o connector is clinging on for dear life. I appreciate I am going to have to cut back the corroded section or wire, my issue is that the wiring section is incredibly short. Meaning I need to extend it somehow. 

What method would you use to extend the cable? Bearing in mind that this cable has a history of getting hot and melting plastic, it is in the engine bay right on top of the engine so it needs to not just be heat resistant but waterproof too. I would solder some new wire in but surely with the heat an vibration will cause it to fail at any given moment.

Extending a 12v wire to glow plug harness