Definitely check the wiring and grounds in the steering column, under the plastic covers. One ground is on the underside, slightly down from where the turn signal stalk is mounted. I found that one when searching for failure of horn and turn signals on one of our 240's.

Another possibility is the tailgate wiring.
Each hinge has a cable running through it, with 4 or 5 conductors inside. Plus a separate ground strap wire also running through the hinge.
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Sven: '89 245 NA, 951 ECU, expanded air dam, forward belly pan reaches oem belly pan, airbox heater upgraded, E-fan, 205/65-15 at 50 psi, IPD sways, no a/c-p/s belt, E-Codes, amber front corner reflectors, aero front face, quad horns, tach, small clock.

It can be caused by inadequate grounding of the steering column to the chassis. The current from the horn feeds back through the wiper circuit. It's fairly common on the 700 series wagons.

Hi,
I would think that perhaps the problem is not directly behind the Airbag, but in the steering column. The horn switch grounds to the steering wheel, and if the rear wiper wire (to or from the switch in the stalk) is bare or grounding, that might lead to horn and wiper going off at the same time.

Remove the cover over the steering column (b/w the instruments & the steering wheel) and look around for shorts/degrading wire. Don't forget to disarm the SRS and disconnect the crash sensor.

I haven't had this problem myself but that is where I would look first. What year is the car?

Good luck!
God Bless
Koos