2004 Blazer Dome lights, dash lights and headlamps with flicker

There's not likely a relay involved in a dome light circuit. Did this problem start after you changed the headlight switch? I would be looking for a wrong, shorted or swapped connection on the headlight switch (assuming that's where the dome light is turned on from).

Well , find the short . videos on youtube showing how to . How to find short to ground blown fuse Finding active short

check out videos on basic electrical testing as well , have charging system tested as well , bad diode can cause flickering lights .

Fault tracing needed;
Test for power
1-To headlamp switch
2-Out headlamp switch w/park and/or headlamps requested on...

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Check the shut off switch at door & the one at the headlight switch. Sounds like a short in one.

The body control module seems to be the culprit here. Have it tested or replaced.

Hi J, The ground connections are on each of the units. that is to say there is one power feed to the dome and another to the instrument panel. From there each door is separately grounded via each of the door switches. If one is rusted, contaminated or damaged and not operating correctly, it will cause the dome light and the door warning on the instrument panel to flicker. regards John

Did you replace the DRL Relay in the fuse panel located on the drivers side dash? There is also one out in the fuse block under the hood.

Alternator, or wiring from alternator.

do not buy without auto electrician

Power runs from the headlight switch down to the dimmer switch, then to the lights. Start by checking for power to the dimmer switch when headlamp switch is on. There should be power in one wire and power out 1 for low and the other for high beam.