The headlamp assembly chassis grounds are to ground rails at the front corner of the fenders behind the air box or washer bottle areas. They can be mildly awkward to get at.

If both high and low beam are noticeably dim on the one side compared to the other then suspect the grounds. Another possibility would be that the reflective coating in the headlamp is noticeably hazed compared to the other side.

Much more likely is a problem at the headlamp connector. It's not an uncommon problem and often becomes chronic once it starts. If there is poor contact in the connector or if you've changed to high intensity, high wattage bulbs then the connector can overheat, fatiguing the little female copper contact fingers which only makes the problem all the worse, even more heat and allowing corrosion to build up. I've seen quite badly deformed connectors from the heat. You can try to pinch the contact fingers together and use contact cleaner -that often provides at least temporary relief. Many people have to resort to cutting off the connector and splicing in a new one. Just be sure to weather seal the soldered splices, like using heat shrink tubing, preferably the self-sealing type. I've got one 940 that's had chronic dim headlamps since the day I got it and occasionally needs the connector tweaked and cleaned, like once or twice a year -I really should replace that connector one of these days.

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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now