[ImperialCrown]

Welcome to Allpar. They flash once? Sounds like a security system or power sliding door/power liftgate or RKE lock/unlock actuation signal?
Any aftermarket alarm or remote start system?
It does sound like it is trying to tell you something. Many of these modules can store fault codes.

 

[ImperialCrown]

With many pre-owned vehicles, it's hard to know what a previous owner has done. A peek under the driver's side dash may reveal added wiring and a black, plastic module for an aftermarket security system. Mopar offered the EVS 1 and EVS 2 that were often dealer installed and configurable to what features you wanted.
An intermittently contacting RKE button in the vicinity of the van may also flash the lights and up close you may hear the locks cycle as well.