I'm about to install an alarm in the Excursion, and would like to use the trunk open feature to unlock only the rear door. That line off the alarm provides 200 mA (-); would that be sufficient to trigger the rear lock actuator? Has anybody else tried this?

 

Sorry it's so long, but there are lots of complexities to this, most of them
are speculative vs. fact.

To your original question concerning 200ma current:

You can always hook up an inexpensive relay to that circuit to drive whatever current
is required to activate the rear hatch unlock mechanism.

but.....

You might also have to disconnect the rear hatch from the existing "lock/unlock"
circuit so ONLY the rear hatch unlocks when you push the trunk open button.
But, that would potentially eliminate the "lock" ability to the rear hatch when you
"arm/lock" the doors - so that would be another side effect to overcome.
Maybe you could hook-up a diode to prevent back-feeding the "unlock" circuit
from the rear hatch activator?

I'm thinking there is more to making the alarm happy with your idea.
And it probably depends (a lot) on the brand of alarm and how it actually
connects in to the vehicle. You'll probably need to experiment once you get it installed.

Typically, in a passenger car the trunk open does NOT trigger
the interior lights so the alarm doesn't go off when you open
the truck when the alarm is on - since it doesn't think the doors are open
by sensing interior light circuit voltage.

I think with most alarms in SUVs when you unlock and open the rear
door w/o disarming the alarm it causes the alarm to go off since the
interior light circuit often triggers the alarm - and you didn't "disarm"
the alarm (which would unlock - usually - the drivers door)
I had an alarm in my '97 Tahoe, installed by the dealer when I purchased it.
Everytime somebody used the key to open the rear door the
alarm went off - THE SAME THING happened when they pushed
the "rear door unlock" button on the remote and then opened the
rear door - THE ALARM WAS STILL ACTIVATED, even though the
rear door WAS unlocked via the key/remote.
- it's been a while since I did this so I could be misremembering (a real word?)
correctly.

Lastly, somebody using the key to unlock/open the rear would potentially
NOT trigger the alarm if you wired things such that the rear hatch doesn't
turn on the interior lights/trigger the alarm.

Let us know if you figure it out though - all knowledge is welcome.

 

the outputs on the alarms are Made to power a relay coil. It will burn out that
function if you try a door actuator on a 200 ma switch! I like miskeys diode idea, Ive used them to steer voltage in the right direction My ford van uses one to keep the battery's separate.