I arrived at the conclusiopn wireing into the loom is impossible. The various computers see the out of orderness and shut down.

First I tried a HHH Advance Parts fuse shubt into the dome light circuit for a pin switch hookup, looks reasonable. No results.

Second, black/pink in the doorwell driver's side carries voltage to backup lights. There are 2 black/pinks and a thick ground. The multimeter sez there is no voltage at 12V in the blk/pnks, a light connected does not light.

Why is this happening and how would an aux rear alarm system hookup to the reverse switch operations ?

Let me know where to find the dome light wire. I tried all at the passenger's footwell

 

Most back-up alarms are tied into the reverse lights at the rear. Pull the light housing and find the correct wire, tie onto it and run it out to the back-up alarm. Make sure the alarm is grounded or if it has a ground wire, ground it.

What are you going to do with the domelight wire? If this is a later model van, most of the circuits are not wired like they were in the old days, but are wired to a computer box which does the work of turning the lights on and off.

 

yeah, we gotta know what you're trying to do for us to be of any help.

for the backup alarm, tap into the wire near the backup lights, as thats the easiest place to identify the right wire. if the alarm draws more than a couple of amps, you'll probably want to power it through a relay, using the backup light wire to trigger the relay, then pulling power from near the battery, using a fuse of the appropriate size.
we can try to be helpful in more detail once we have more detail, but we're kinda shooting in the dark how it is, not knowing what year econoline or what exactly your goals are here

 

OK, I'll move to the back. WHY ?

I have 2 Optimas/inverter back there balancing the gas tank, over a set of Bils and a Hellwig rear bar.

An 8 GA cable runs back from the Odyssey batt past the Kenwood mounted on partition and 3 ! Omega alarm units. Spaghetti

Stereo and inverter do not mix. NOISE. The inverter should be towed.

Grounds and the dumb lite have me stumped.

I have tried all the brown wires at the computer without results. I was advised a relay is necessary to allow all pin switches into the alarm circuit. Driver's door unlock only is the choice for Ford's key fob controller: this may be an obstacle.

All instructions are use the brown. No one sez there are problems.

I use the van for animal behavior field research out of parks n camps so mounted extra lights, a spanish voice alarm and a backup whistle beeper for children and drunks.

A Bosch relay pamphlet is in the shop, somewhere. I remember 3 relays are required to branch off to 2 units from one signal.

Tried dioding from a front compartment ground switch to rear relay to the 2 backup alarms but no go there, one at a time. Not hooked into the loom.

 

and, I found part HHH at Advance, a well done fuse shunt. EUREKA ! Place fuse shunt into Domb Light fuse holder, run shunt to alarm system with diode.

ZERO !!

How ? Voltage goes thru fuse to light. Ground pin, voltage flows, alarm goes off. Nada.

Vood.