So I'm now regularly getting the TRAC/ASC/ABS warning lights go on, as opposed to only intermittently in the past.

I've already opened the ABS box and solder was fine.

I've cleaned front left and right wheel speed sensors and that did not seem to help (nor should it make a difference).

I've also used contact cleaner on them and the plug going into the ABS box.

So, what are the next steps? Get new left and/or right wiring harnesses?

 

Try the test and check sheet on my page I think the 1999 & 2000 are the same if not it will get you started. Chances are you have a poor connection or a bad sensor.

Link JagRepair.com - Jaguar Repair Information Resource

 

Thanks, forgot about that chart! Will check resistances and see what comes up.

 

You need to get the codes. On my car I had the pump circuit code and left front sensor code. Switching the front sensors and harness did not move the fault to the right side, and testing resistance at the module connector showed the same for all four wheels.
I sent the box Module Repair Pro and the problem was fixed.

 

+1 on the codes now that there is a constant error message. Because of the constant message it is probably not a factor but you should make sure you do not overlook the complete sensor cleaning instructions. I am unclear if you have done so but you should ensure the ABS cogs on the inboard sides of the wheels, what the sensors read, are clean. I literally had to go through 1 1/2 cans of brake cleaner to accomplish this. The rears are a little tricky because they are covered.

You may have done this but I only mention it because I inadvertently skipped that step and saw no improvement by just cleaning the sensor and electrical connectors. Once I completed a thorough cleaning the errors were gone.