Looking for some advice after searching the internet over and over for answers.

The STOP light on the dash started to flicker on and off around a year ago. For 6 months now it is on constantly when the A/C is on, but goes off when I switch the A/C off. The passenger side radiator fan kicks in and out on what seems to be a high speed when the A/C is on, but not when the A/C is off. The temperature gauge stays around 75 degrees all the time, except if I'm towing, it can rise if going up a long steep hill with a heavy load.

Here's what I have done so far:

I cleaned my bitron units connections and even tried another 2nd hand unit, but no change. I suppose there's nothing saying that if it is the bitron, then the 2nd hand unit could have been faulty too?

Verified the radiator fans are not seized by moving them with the engine off.

I have cleaned the connections on the 3 relays behind the grill. If I take them out one by one, I can still only get the passenger rad fan to work.

I have checked the brown sensor on the thermostat housing. If I pull it out when the car is running, the passenger rad fan kicks in after a few seconds. I bridged the terminals on the connectors lead, but it did not change anything.

I have cleaned the connector multiplug on the left hand side of the radiator.

I have cleaned the earth just below the battery housing.

I am now left to think it can either be one of the following: replace the 3 relays behind the grill OR buy a new bitron OR replace the right hand side radiator fan.

Can anyone offer any other fault finding tips or suggestions?