Why does the backup alarm sound without the levers being in reveverse? It works as soon as the power wire is put on the terminal

Remove the aftermarket alarm system. It is dieing. Soon you won't be able to even start the van. Repair wires as you go.

It comes from the transmission range sensor. When you put the lever in reverse the sensor on the transmission relays this to the backup lights.

the valet key doesnt have a transponder chip in it, so it will allow physical turning of keyways and operations, but will not disable the alarm and often wont disable the immobilizer.

sorry to say i think you will need to get an autolocksmith , or main dealer to get you a functioning key with transponder chip mapped to your cars rfid system that will allow alarm to be enabled /disabled

This happens in the shop all the time. It is not necessary to remove the alarm if you have the remote for the alarm. Simply hook the battery up and press the button to turn the alarm off.

Might want to check the lens for the reverse sensors to make sure they are not damaged or dirty.
Next is to check the wire from the back to the sensors, some times the connectors become corroded and if the sensors are not "seen" by the PAM ( Parking Aid Module ) the system turns on, then off right away.
The sensors remove from the back of the bumper ( they do not pull through to the outside ), and the wiring harness is fairly easy to track. Check each sensor and the connector for the sensor harness.

2 possible reasons. 1 is that the alarm is not able to function correctly with voltage that is too low. 2 is that the battery theft part of alarm is being triggered. This alarm is not factory OEM.

These type of alarms used to have a secondary battery that powered the alarm solely (small battery, usually in same housing as alarm). It is possible that the backup battery has failed.

Alternatively, you can have a mechanic or technician wire in a small sealed battery to the alarm that is connected to the main battery through a diode. That way, the backup battery will be charged off alternator, but will not discharge except through the alarm. In any case, even the backup battery will fail eventually and you back to same problem.

If it's a cheap aftermarket alarm, check for a red wire coming off the hot side of your battery with an inline fuse. If it's not, try to locate the alarm "computer" under the driver's side dash or nearby your fuse panel. You'll find a black box with a wiring diagram glued to the side, normally strapped to a wiring harness or other fixed element. If this is the type of alarm you have, you'll need to trace each wire coming from the alarm computer and unclip each fastener that's clipped to your vehicle's factory wiring. The trouble with these types of alarms is that they can get power from a number of sources (stereo, interior lights, etc.). Good luck with this one!

First check for power at the light sockets and make sure they are properly grounded. The neutral start switch should also be the backup light switch (located on the transmission at the shift lever). If vehicle is starting in the correct shift positions, likely it is adjusted properly. However, have someone watch the backup lights and move shift lever through the gears. if while moving lever lights flash, switch may be slightly out of adjustment. Pink or dark blue wire should be power supply to switch, light green should send power to lights.This is powered by a 15a fuse on the fuse panel.

Sounds like 2 problems. One a low battery. When you used the laptop you might have drained the battery of the vehicle. The alarm sounds like broken wires in door harnesses between the doors and frame. Remove the rubber seal beween the door and body of the car and look for cracked and broken wires. If they are broken or touching it can cause many wierd problems. Once you locate it which is easy you can show a certified mechanic and he or she can repair or replace the harness.

Yes this is an Alarm issue that's why your vehicle is shutting down as soon as you press on the brake pedal. Is this an aftermarket alarm or stock alarm? Sounds like aftermarket alarm to me and if it is, the way fix this issue is as soon as you put the red n black terminal on the battery and the alarm goes off press your alarm key right away to turn it off. If this doesn't work put your key on the ignition after you have installed the battery and just put it on the "ON" position engine OFF then click on your alarm remote once again.