Its unlikely to be worth your expense to buy a tap, they can be quite expensive, just cover a bolt with oil, run it in and out and wipe it off, repeat a few times cleaning the thread out with brake cleaner or even wd40 would do the job.
I only have RAVE that covers soft dash otherwise I'd post the wiring diagram, but I'd guess the problem can only be the transducer or the speedo itself, unless you have a chafed or corroded wire between the two. Have you had the binnacle out and looked at the multiplugs & PCB?
If it has a transducer on the tbox then the speedo isnt driven off abs signal. A relay is just a switch for high current using a low current signal so there wouldnt be one for the speedo either
I may not be correct but mine is definately cable. I also remember that the cable to fit the BW box has a larger square drive than the LT230, I had to file it down when I switched
That sounds like a lot but if that what rave says...
Cable should be adjusted so there is a small amount of slack with hand brake off
As mentioned this type of handbrake is rubbish, Ive had BMWs with all new parts still not hold on a hill :(
You would have to put it in difflock ;)
I would also add to check your tyres for lumps, perishing or mis-shape.
You would know if it was death wobble, it feels like the whole thing is disintegrating around you 😁
Very early ones had the 3.5
They are only prone to issues mostly when neglected, if you keep it well serviced and replace anything iffy before it becomes a problem then they are usually ok
Still seems like a lot of work doing the swap tho, as always complete donor vehicle is the way to go
I put genuine ones in mine and they pushed in easily with a piece of tube, I didnt even have to tap it. No leaks yet 🫣
However mine wasnt leaking before, I replaced as I was doing balljoints so seemed a good time to do them
Agree with above diag would give a clue, until then there are simple things like check for air filter blockage and I believe CPS wiring is prone to chafing