Bad contacts on the fender-mounted relay are possible, as B.C. suggested. That would be the "incoming" lead, coming from the headlight switch. The output side of that relay seems to be OK. I believe the incoming is a yellow wire, and it's pretty thick.

The main reason for the introduction of the main headlight relay (at driver's kick panel) in the '86 and later cars was the too-frequent failure of the headlight switch on the dash. It carries a lot of current especially when you use the high beams.

So look into that switch if the relay doesn't answer your problem. The output wire at switch is the same one that's input at the relay, thick and likely yellow.

Also you should find 12 volts going into the switch. Not sure if that will require the ignition switch to be in some particular position but you will figure that out.

For your parts search:
According to fcpgroton.com ("Relays and Switches" page)
'76 - '84 used either a 3-pin or 5-pin (blade) switch.
'85 used only the 5-pin version of the above two.
'86 and laster used a different switch.
Priced at $10 and $13, not too bad.
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Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors, quad horns, tach, small clock. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.