New heater plug harness & maybe to replace glow plugs
 

My glow plug lead is starting to "melt" where the threaded terminal is..... its done 161k miles and each time you switch the ignition on, the rust/corrosion between the post, nut and terminal causes resistance ...makes the post get hot and the insulating plastic burn.... anyway, no issues with that, one from eBay for £16 or brand new for £45+VAT.... though.... my question is.... while I am stripping all this out, and given that new (genuine) glow plugs are now half price at £10 a go.... is it worth renewing the glow plugs anyway.... I mean, its 14 years old and done 161k mile.... given I have to strip everything out to replace the harness, is it worth renewing the plugs? what is the life expectancy? ….note that this last winter when starting from cold it did sound a little "rough" even after using the plugs... so am thinking one maybe down anyway.... so for £10 a pop might be worth doing the whole lot...

Expected life expectancy of glow plugs are 100,000 (U.S.) miles.

The following are some of the signs that you will notice when it is time to have your glow plugs replaced:
The diesel vehicle is hard to start during cold weather
There is more smoke coming from the vehicle than normal
The diesel engine is misfiring on a regular basis
Decreased fuel efficiency

WDF, for the cost of the glow plugs, I would say to replace them. It sounds like they are at a minimum getting weak. Besides, if you are half way there, it will make any work you would have done later, that much easier.