Many years ago my 1994 940 had melted fuse holders 6 and 16 as I frequently use the blower and AC. I did bypass them with fuse holders before, but the holders themselves became warm during operation. After some years they also melted. So there must be more than what meets the eye here.

I ordered a used fuse block from the eBay, took some time to install it (as per John Sargent's advice) and until today its still doing ok. And the fuses on the new (used) block runs cooler than when they were on fuse holders.

I dismantled my old melted fuse block and realised that there's some kind of heat dissipation method used at the high current connections. What we see as fuses 14, 15 and 16 sharing the same supply also means they are sharing the same heat dissipation too. They share the same metal power rail within the fuse block. They are all thermally connected!

You didn't mention which fuses are affected in your car but if they're the high current ones I think you're better off replacing the entire block than splicing in fuse holder.

Regards,
Amarin.