Where is the daytime running light module on a 1990 Buick LeSabre?

The 1990 Buick LeSabre did not have a daytime running light (DRL) module in the modern sense. Daytime running lights as we know them (a separate, dedicated system) weren't standard equipment on cars of that era.

If your 1990 LeSabre has what you *think* are daytime running lights, it's likely that the headlights are simply dimmed slightly (or perhaps the taillights are on at a lower intensity) when the ignition is on. This was a common way to improve visibility in some vehicles of that period. There's no separate module controlling this; it's integrated into the existing headlight and/or taillight circuitry. Any modification to this "DRL" system would involve working directly with the vehicle's wiring harness and light switches.