Bill,
You can check for corrosion on the relay pins and the stabs connections under the fuse/relay panel.

If you have the red Hella Gluhlampenkontrolle, Volvo part number, 1362370, and to verify that it is working correctly, pull it and make the following checks with an ohmmeter, test light, or buzzer. This is from a 1995 940 wiring diagram but is should be the same for the 740. I played around with a spare and at least one small alligator clip on a test lead would be most helpful.

Reed switch, pins K and 31 should be open.

The following should have continuity.
The wiring diagram shows "54S" and the relay had "54".
From pin 54 to coil pin 54L (Left brake light)
From pin 54 to coil pin S (3rd brake light)
From pin 54 to coil pin 54R (Right brake light)

From pin 57R to coil pin 58R (Left parking/tail light & Right parking light, rear)

From pin 57L to coil pin 58L (Right parking/tail light & Left parking light, rear)

From pin 56b to coil pin 56bL (Left low beam)
From pin 56b to coil pin 56bR (Right low beam)

Thats all 13 pins. Good luck.
--
Tom F Four 940s, RIP '78 244