RS485 signal switching

use a thansistor to realize a compartor is OK.

I would assume that switching loads in and out in this manner would introduce glitches into the line. If you're using RS-485 like most people and putting half-duplex asynchronous serial onto it then it should be pretty robust to glitches.

"Pretty robust" doesn't mean "immune", however. If I were designing a system with this sort of thing in it I would pay close attention to the communications protocol built on top of my RS-485 bus and asynchronous serial transceiver. I would make sure that it has a means of error detection and correction, probably backward error correction with a checksum, acknowledge and repeat structure.

Most people use RS485 with such protocols anyway since a bus with more than 2 nodes on it would at the very least need some sort of arbitration. So I assumed that's the scenario described by the OP. But yes, make sure you use the appropriate protocol for the job.

If you use a relay have passive pull up/down resistors on the line to put it into a known state whilst the wipers are flying. AFAIR the pulls are A to -ve and B to +ve.

The simplest wire is multiple RS485 line drivers and Enable/Disable each as required.