1992 400SE

I recently had my vehicle boosted by a "well-known" "experienced" (at least I thought) company. Turns out he hooked up the battery terminasl in reverse.

After getting it right, with the vehicle running the ABS light is constantly on. I checked all the fuses and they are fine. I'm guessing it could be a relay???

Where can I find the relay(s) controlling the ABS, or what else can be tested? Or is there a simple reset.

I disconnected the battery in hopes of clearing/re-setting it, but without luck.

Thanks in advance.