hankster72
If the tach. do not not function, it could be a poor connection, or a pinch wire coming from the engine module that manages all the engine components.
On the fuse box panel ( at RH of the motor under the hood ) is a listing of the fuses and also at the passenger interior up of the foot well under of the carpet an other fuse box ( call BCM ) were all the wiring harness circuits are controlled.
The cluster or instrument panel are to look for defective fuses.
Tachometer problem is usely not an issue debated on the site, connection could the problem, however it could happen that it would be defective.
An expert is the solution, a trust Cadillac dealer is the one that has more chances to be the one,
Also a trust Chevrolet/Corvette dealer could be one as the corvette cluster is very similar.
They have the tools and trained mechanics.
Welcome on this site, the cadillac XLR is a great car however its electrical system is already 17 year old, so it would needs to be serviced.
Enjoy this one of a kind car.
IAN