I have a 98 Ranger with cargo lights. There is no info in the owners manual on how to turn them on. Instrument panel dimmer in either direction does not work, no switch on the dome light. Light comes on when you open the door, so does the door ajar light. Took off the bl cl assembly, put in new bulbs all wiring looks great. So whats the story. Oh also checked the fuses.

Let's see......if there is no provision on the headlight switch assembly plus no switch on the dome light and the owner's manual doesn't mention such switches, it's time to use deductive reasoning and common sense.
Open the door to see what's in the bed at night.

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On my '97, the cargo lights are activated by turning the dimmer switch up to the extreme high position, past the "click" detente position.

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1997 4x4 Mazda B4000 SE, Ext.Cab

Roll the dimmer switch to the top and past the detent, that will not only actuate your cargo lamps, but also your dome light manually with doors closed.

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'01 XLT disguised as a '03 Fx4 Level II.

I have tried that, so evidently my dimmer switch is bad. It goes all the way up to full brightness on the dash lights but no detent.

I evidentally wasn't pushing it up hard enough, I now have cargo lights and the dome light comes on with the doors closed. Very Happy. Thanks to everyone on the forum who helped.

Nice.

Yep....push her past the detent.

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'01 XLT disguised as a '03 Fx4 Level II.

Same situation with my 2003. Much harder to push past the detent than my 2000 was.