Headlights don't work when engine running

2008 Tacoma. All lights work when engine off. Headlights don't work when engine on. Fuses are good and just installed new headlight relay today. Ideas?

 

to confirm:
key on but engine not running, headlights work
get the engine running and the headlights stop working

2 thoughts:
1) a loose connection that is not making good contact do to engine vibration when running
2) if the headlight control goes through a computer module that module is f'ed

 

with the key off all lights work. Haven't tried lights with key on. But yes, when running all lights work except headlights. Vibration is a possibility, but don't think likely as there isn't much.

 

Which headlight relay did you replace (there are 2)?

 

Looks like 2008 Tacoma only has one headlight relay

 

It does not.
Have a skilled electrical mech assist you...

 

Head and Dim relays

 

Thanks. I've now swapped both relays and still doesn't work.

 

More internet research and I'm guessing it may be due to a failed after market alarm system control module. Karr brand. Another symptom besides the headlights is the horn beeps when starting the engine. That's been happening since I bought it last year.