We need to tap into two wires going to the headlight switch.
Yellow/Blue wire for illumination wire on the head unit
This wire goes to the headlight relay so it will have +12V when you turn the headlight switch to the On position (assuming your DRL’s have been disabled).
This wire needs to be connected to the orange Illum wire on the head unit. The unit needs to know when to dim the display so if the wire has +12V, the unit switches to night mode.
Green wire for the Acc wire on the head unit
There are 4 positions for the ignition key:
LOCK
OFF
ON
START
The red wire in the Metra harness is supposed to be connected to the ACC wire on the harness. However, this wire is only powered when the ignition is in the ON position. This means your engine needs to be running in order to be able to use the head unit. You can turn the ignition to the ON position without having the engine running but this bad for the ignition and other engine parts. What we need is a wire that is powered when the ignition is in the OFF position.
That’s where the green wire from the headlight switch comes in. It has +12V when the key is in the OFF, ON and START position and 0V when in the LOCK position. So the OFF position in the Saab 9-5 becomes the ACC position found on many other vehicles.
Procedure
Take out the fuse panel cover on the left side and push out the headlight switch from behind.
Slide out the red part on the harness and remove the head light switch.
Use wire splice connectors to tap into the two wires
Route the wires under the dash. The panel is secured with only 3 screws so it is easy to do.
