Hmm, that's the same code I'm using, and I thought it was 30 seconds, I even got in another Lada and drove it around hitting the horn rapidly in frustration while waiting on the other.
I'll go get a stopwatch to make sure though and doublecheck my code, if anything I'll copy yours straight to the ost.con file.
As far as the glitch, I'm not too worried about that. I just want a cheap guerilla warfare tactic, I guess you could call it that.
Maybe put the carhorn to a more inconspicious key so not everyone and their grandmother causes nuclear Ladas to detonate. 
Edit: I got it to work, and it's right around 30 seconds, when it works. The problem is sometimes the timer sets....and sometimes it doesn't. I've also had trouble before out of some vehicles (and car horns, ironically) not wanting to register clicks, such as calling for arty in little birds, honking some civilian vehicles horns, etc. but I never have trouble with guns or tanks. It's not quite related to the problem I was first faced with ,but anyone have any suggestions?