My first thought would be check the ground wire second check and be sure they did not swap fuse to a different plug I had a customer that did that so alarm wouldn't sound. Also look into a smart alarm ?
The smart alarm has a low alarm until it senses noise then goes to loud
If you cannot find issue with the wiring I would suggest looking into the plug under the relay as well as fuse I have found loose pins in past
Take your meter to the plug on relay and check
For voltage and negative to relay as well as output to alarm if still in limbo please let me know I am a retired tech (43years ) over 22 on Linde