if you install an aftermarket alarm and incorporate flashing running lights into the alert along with the blaring siren that might have car thieves run the other way. I'm guessing the neighbor's cars had no alarms at all? I would and have recommended the Code Alarm CA1055 as they're just plain dirt simple to install and operate. A good source is Crutchfield:
https://www.crutchfield.com/p_220CA1...rm-CA1055.html
The Ravelco is a very good choice as well, another source of deterrent that if carefully concealed from immediate first peek inside the van or vehicle will be more than the average car thief wants to hang around and try defeating.
BTW DIY Ravelco's can be found new in boxes for under $100 on eBay.
HTH