Yes, heat will locate the problem, usually the resistance will fall when you get close to it. You don't need much heat. A soldering iron or lighter flame on a leaky pot will get a response almost immediately. Another possibility is that the cable has been damaged somewhere other than a termination, which you will only tend to discover once you have ruled out the terminations.
As I understand it you can access the joints but with difficulty and cost, so you want to try to narrow it down using only the accessible ends of the cables while they are all interconnected. I had a similar situation where I managed to localise the fault to one box, by resistance heating the cables. Using a low voltage high current transformer I pumped two or three times rated current through the Pyro from each tap-off to the next, while meggering the complete run. By identifying the combination of conductors that triggered a change in insulation when heated, we ended up with only one box under suspicion, which once exposed, revealed a badly made seal where the disc had never seated properly in the pot.
More recently I had an experience with a lighting installation that had so many bad seals that any attempt to localise faults failed miserably, and we had to rewire it completely.
