99 GMC Sierra headlights & running lights won't work!

Headlights Circuit Description
The headlights are controlled by the body control module (BCM) via the AHL Relay when the headlamp and panel dimmer switch is the AUTO position. When in the AUTO mode, the BCM receives a signal, (voltage) from the ambient light sensor, based on the voltage of the signal of n the sensor the BCM turns the day time running lamp (DRL) lights OFF and the headlights ON. The headlights can also be turned on by placing the headlamp and panel dimmer switch in the head position.
The relay coil is fed by the headlamp and panel dimmer switch or the BCM and the battery B+ buss bar in the underhood bussed electrical center (UBEC). The BCM supplies current to the relay based on the input from the ambient light sensor via CKT 352.
The relay output feeds the LT HDLP and RT HDLP fuses to supply current to the low and high beam lamps on CKT 1340 and CKT 1240. The ground for both the low and high beam lamps is supplied via CKT 524 and CKT 525 to the dimmer switch. The dimmer switch uses CKT 1850 for a ground to G203.
Some circuits are monitored by the BCM, and upon failure may set a DTC.
Park/Side Marker and Tail Lamps
The park/side marker and tail lamps are split between the following 4 fuses:
?€¢
The FR PRK

?€¢
The LR PRK

?€¢
The RR PRK

?€¢
The TRL PRK

These fuses receive voltage from the park lamp relay. When the headlamp and panel dimmer switch is in the auto position, the body control module (BCM) controls the relay by grounding CKT 1080 based on the input of the ambient light sensor. The park LPS fuse supplies battery positive voltage (B+) to the relay at all times.
When the headlamp and panel dimmer switch is in the PARK position, the relay is by passed through the INT PRK fuse. Current flows from the PARK LPS fuse through the headlamp and panel dimmer switch to the INT PRK fuse in order to feed the following fuses:
?€¢
The FR PRK

?€¢
The LR PRK

?€¢
The RR PRK

?€¢
The TRL PRK

Park Lamps Inoperative

Step

Action

Value(s)

Yes

No

1

1. Turn the ignition OFF.
2. Connect a scan tool.
3. Turn the ignition to the ON position.
4. Using the scan tool, check for any DTCs in the body control module (BCM).

Does the scan tool show any DTCs ?
--
Go to Diagnostic System Check - Body Control System in Body Control System
Go to Step 2

2

1. Turn the headlamp switch to the park position.
2. Connect a test lamp from (back probe) terminal B of the headlamp and panel dimmer switch to ground.

Does the test lamp light?
--
Go to Step 4
Go to Step 3

3

Locate and repair the open in CKT 240 (ORN) between the switch and the mid bussed electrical center (MBEC).

Is the repair complete?
--
System OK
--

4

Connect a test lamp from (back probe) terminal A of the headlamp and panel dimmer switch to ground.

Does the test lamp light?
--
Go to Step 6
Go to Step 5

5

Replace the headlamp and panel dimmer switch. Refer to Headlamp Switch Replacement .

Is the repair complete?
--
System OK
--

6

Connect a test lamp from the INT PRK fuse, located in the left bussed electrical center (LBEC), to ground.

Does the test lamp light?
--
Go to Step 7
Go to Step 8

7

Locate and repair the open in CKT 74 (LT BLU) between the MBEC and the UBEC.

Is the repair complete?
--
System OK
--

8

Locate and repair the open in CKT 74 (LT BLU) between the headlamp and the panel dimmer switch and the LBEC.

Is the repair complete?
--
System OK