Eberspächer auxiliary heater fan relay in Volvo 855?

I bought a built-in Eberspächer Hydronic (?) stand heater for a 1996 Volvo 855 TDI. The remote control TP48 works. The heater turns on and heats the engine coolant well, but the interior fan does not work, meaning the interior is not heated. I believe the heater was retrofitted and not originally installed. The fuses are intact, and the heater fan works flawlessly within the "normal" heating system. Apparently, the retrofit kits have a relay for controlling the heater fan. The problem could be there in the simplest case. Unfortunately, I don't know what the relay looks like or where to look for it. The image http://members.aon.at/msemrad/A160/Bausatz.jpg shows a newer installation kit. The only thing that looks like a relay is a small black box on the right. Can someone help me with the relay? Where to look, what size?

I accidentally posted in the wrong forum. Sorry.