Maybe if you hold the button in long enough, the truck turns into a Transformer.

Seriously though, I'd bet a small stack of $20's that it's some kind of alarm system that some shop talked the lady (original owner) into buying. Some of those "systems" are as elaborate as just a fake blinking light. But this one probably goes to some add on module. Have you looked around under the hood for an extra box?

 

I'll go with an aftermarket alarm system also. Get back under the hood and see if you can find a horn or a control box. Or take it to Best Buy or one of the alarm installers and see if they can trace it out.

 

Have seen them installed by used car lot dealers. It's some type of security system I believe. The used car dealer should be able to remove it.

 

Reminds me of an electric parking brake release I saw once.

 

Contact original owner.