The horn on my wife's 98 S90 became intermittent and she wanted it fixed. In the process, I learned a few things that might be useful.

1) The 18 year old plastic parts are becoming more of an issue, but some can be worked on. I've been using an epoxy made for plastic, sold by Pep Boys, etc. that does seem to help put parts back together. However, it really works when you can add epoxy such that the epoxy supplies additional support. Some parts, like the high brake light outer housing, that simply break in pieces are beyond help

2) After schematic review, I checked the ground signal to the relay that is supplied from the steering wheel mechanism (car does not need to be running). When that showed bad, I yanked the steering wheel. Be careful and follow instructions, the task isn't too bad.

3) I found the Horn Contact Assembly to have several problems. The first was again the plastic, one of the lower mounting tabs had broken off on the 98 car and the upper mounting tab had done the same on the 97 donor car. For both parts I used glue/epoxy to attach the broken tab and then added epoxy to reinforce all the tabs. The mounting tabs only hold the assembly in place, both horn and air bag electrical connections are via wires.

4) The real problem with the assembly was that the grease on the sliding contact surface had simply dried up and formed a hard surface over part of the rotary contact. The dried grease did not allow electrical contact with the spring loaded contact on the back of the steering wheel, hence no horn.

5) After cleaning the contact surface with solvent and a polish with 800+ paper, the question became what grease to use. Common / conductive greases that I could find were all petroleum oil based and prone to dry out over time. Therefore, I used a dielectric grease that is silicone based. Even though the dielectric grease can act as an insulator, it will allow good contact between the spring loaded contact and the sliding contact surface. The result is a functioning horn, without purchase of a new and expensive Horn Contact Assembly.

6) I also found that the upper and lower steering column covers had problems with degraded plastic in both cars. Several of the sockets for the cover attachment screws had sheared because the oil in the plastic had dried out. I was able to use the upper cover from one car with the lower cover from the other, after applying some of the epoxy to reinforce the screw sockets. I stopped by the dealer and found that the covers are still available, a bit over $50 for the pair, and I ordered new ones. I plan to also add epoxy to the new covers before they go in the spare parts pile.