Ok this might be a little long winded forgive me but wanted input.
2003 XKR convertible 77k miles

This car had its security module replaced. I have 2 modules one was the new replacement one was used for tested at the shop I had do the work

I got the car back a few days ago GOT A NEW battery (not the right size (not long) but according to the shop same by all other accounts) and did not notice any issues. Drives great lt had all abs sensors replaced, engine sensors and removed all check engine abs warning repaired. The first night I drove it I got a check rear lights notice on the screen. I stopped and looked didn't notice any light not working.

I went though and replaced fuses in the car after I noticed the after market radio wasn't working. Also took it out and just checked it. Appears to be a radio issue. Some time later i noticed rear lights were on and side markers were on even though the car was off and locked.

At this point i call the shop and he said lets do a hard reset could be the new battery. Did the hard reset and didn't fix the issue. Put the car up for sale not joking. Other weird issue with replacement security box let's call it box A.

Next day I noticed the shop had left a another security module and he said it's my shop test box because you can't program remotes with it. We'll shall call this box B.

I opened both boxes to check for burn marks ot just overall if they were the same on the inside. They share the exact same part number but they are not the same.

Box A looks like an older box electronic wise. Box A shows the following symptoms. Tails lights and rear side markers are always on. Quirks with box A that exsited before tail lights always on issue or noticed it) locking the door with the key was fine. Remote would not lock or unlock any door, and using the key works but unlocking with the key the top would go down. Just a quick unlock (not the long hold) will cause this. And sometimes locking it would cause the top to go down. Interior would stay on even after starting the car for like 5 mins doesn't sound bad but annoying. Brake lights, blinkers work.

box B. This one appears to be newer after inspection also noticed because I'm a computer guy the ribbon cable going to the wireless card looks to be installed wrong meaning i bet I can fix the remote programming. I digress sorry. Box B I have no tail lights nor rear side markers. I also do not have rear brake lights. The cyclops brake light on the trunk does works. Car locks correctly, well passenger door doesnt but more testing to do there. The good things are the Interior lights work as they should. Indicator beeps are back. Door lock no longer makes the top act weird.

shop tech is going to looknat a diagram tomorrow. He wants to check to see if there is a diode somewhere in the harness. He thinks the xk does have one.

Could this be bulbs, wiring, or both boxes shot? I don't wanna go crazy on this because I'm selling the car but I also want to hand over a car without an issue like this.

 

A bit of a boring answer for you, but the only way to debug this is with a multimeter and the wiring diagram. See what is going in to the SLM and what is coming out. The wiring diagram will tell you which lamps go through the SLM and which ones don't. Try testing with the SLM plugged in and also disconnected and see if you get the behaviour you expect. This will help pinpoint where the fault(s) are.

 

I tracked down the issue it was a bad BCM. I replaced the BCM but now the car won't start. Is the key programmed in the bcm? If so how do I fix this? I only have 1 key. Fob works key doesn't.

 

See the attached document.
Replacing modules is not straightforward on the 4.2L.