No-crank/no-start... security system?

So I shouldn't be complaining about this too hard because it's the reason I got the truck so cheap but I've got a no-crank/no-start condition that's a bit perplexing.

Truck has an aftermarket security system of which I snipped the horn but it's still going off whenever I put the key in (silently thankfully) and I halfway suspect it's disabling the starter. However, my mechanic buddy took a look at it and he was convinced that the starter was going bad. Apparently the truck started up for him without much issue. So I slapped in a reman starter... nothing. The dash lights come on, electrical stuff all works and the security system blinks but when I turn the key it doesn't click or attempt to crank at all.

Do you guys have any opinions on what this could be? Are there any more diagnostics I can do to pin down the issue? From where I stand next order of business is ripping out that damn security system.

 

So, just to update this truck is running now after taking the security system out AND replacing the starter. We're like 90% confident at this point that it was a combination of the security system + a bad starter. Very confident the security system was in play and like 80% confident the starter was also an issue.