[targa80]

If I had this issue I would consider the following while reviewing the schematic diagram of the circuit.

1. The relay overheating is due to a higher than normal current draw within the relay portion or the power window circuit.
2. Test the window motors separately to do this I would add an ammeter in the circuit at fuse 21 (25 amp) to check the current draw while activating each of the three window switches separately. The higher motor current draw would be the side to do a detailed inspection of all connections.
3. Try replacing the relay and/or the individual window switch’s after testing the current draw.
4. Check each of the connections in the circuit for corrosion or heat damage including the ground connections
5. The relay, fuse or switches, window motors could cause the issue due to the age of components and the heat being generated while active.