My interior light, el. windows, central locking, sunroof and so on didn't work on A6 2.8 1998. The pboblem was the ccm module under drivers carpet. Just took it out and cleaned all the coonectors and the cirquit plate and everything is good now! Just if it might help.

 

If your bcm is failing/failed it can cause all kinds of issues like these, ive worked on several G35 infinitis that had all sorts of random problems at once, each time the bcm was at fault. the other thing is check every ground wire you can get to, especially the ones to the bcm and central locking. poor grounds will also cause a world of trouble

 

Id have to agree with some sort of issue with the bcm or body control module. On the audis its called the comfort control module. That module activates the hazard relay to flash lights for locking unlocking, and it is the input and output for both the door locks and the door switchs for open and closed position. Get that module checked for faults.

The horn is probably a serperate issue. Its not uncommon for both horns to be bad. I would check for voltage at the horns with the horn button depressed. If you have good ground and power you need a couple horns. Otherwise you may have a switch or relay issue.

You also may not be far off with the car coming from the bottom of a lake. That module is under the drivers seat. So a foot or 2 of flood water could have gotten to it.