1985 Cadillac EL Dorado Air conditioning system

The entire air conditioning system is never replaced. That would cost more in parts and labor than the car is worth. Individual parts are replaced as necessary. Unless you can tell me exactly which parts were replaced and what additional services were performed, I would have to guess the evaporator was replaced. That is inside the heater box under the dash. It's possible a seal wasn't seated properly, but more likely this is a result of an enthusiastic car wash. Water and soap can get blown into the same place where rain can run into, and where the water that condenses when running the air conditioning system collects. That water drips into a plastic pan, then runs out a drain tube onto the ground roughly under the front passenger's feet. I suspect the soap in that water got aerated and blown through the system.

If you don't have this problem when driving in rain, it is likely there really isn't a problem.

None of those parts are in the heater box inside the car, so you're right to think nothing has changed in that respect. The next thing I would look at is the drain tube under the hood. It is fairly common for them to get plugged with debris, then water builds up in the drain pan. The more common symptom is that water overflows, especially when turning corners, and appears on the front passenger side floor.

If that drain is not completely blocked yet, I could see water and soap building up faster from a car wash than it can drain out. That soap won't cause a problem yet, but the blockage will continue to build up until you have more water problems. Best is to address this now.

Look for a 4" long rubber hose with a 90 degree bend in it, hanging down from the firewall on the passenger side. Squeeze the end to open it up and let any debris wash out. Normally the trapped water does that, but if yours isn't that bad yet, the blockage may not wash out. There's a wire ring holding that tube on. Squeeze the ends of the ring, then pull that hose off. Check it for blockage. If there is none, stick a pencil into the plastic spout the hose was attached to. That should also dislodge any blockage.