Van keeps thinking it's being stolen.

First time poster, I'm looking for some advice on what to do to fix it.
I have a good parts guy, but I don't have the $$ to let the dealers poke around to try to figure out what is wrong.

So my Van is a 96 Plymouth Grand Voyager. (3.8L if that matters)

There are some strange things with it.
For starters when I try to unlock the doors with the key, the door lock motors fight me and try to lock it back up.
Also the sliding doors do not lock when I press the door locks (I think this is a separate issue.)
And sometimes when I open the door and put the key in, the van car alarm goes off. (Once I open the door, if I use the internal door switch to unlock all the doors, the alarm won't trigger. Most of the time.)

We were recently doing some work on the van (replacing the dash console) when I discovered a 3rd party alarm / remote key entry device attached to the van. Looked it over called their support and discovered that their install taps into all the same wires but doesn't cut any. I removed the box (thinking that it would solve the problem) but there was no change. I'm thinking that the 'floating' wires and the installation of the 3rd party box messed something up. I just don't know what.

Any mechanics that have any ideas? Maybe I need to replace the BCM? What do you think?

Thanks for any help!