I wear a Leatherman Pocket tool in a pouch on my belt everywhere I go - very handy to have. If you establish earlier that your character has such a thing - using one of the blades to cut something or the pliers/screwdriver blade to fix something - then at the "hanging from the cab" scene (s)he could take it from its pouch and use it to smash the window.
The window's likely to be safety glass but a good crack from the end of a metal Leatherman would cause it to shatter.
You don't have to go all out to explain a pocket tool (unlike a gun or a laser or a convenient brick) - (s)he's a traveller, it's a handy thing to have (knife blades, screwdrivers, pliers, bottle/can opener, folding scissors etc) and is compact and unobtrusive - its uses justify the negligible weight - but it's heavier and more substantial than a pocket knife (I'm sure it'd easily smash the side window of a truck. Probably even the windscreen)
You could even use it without the Checkhov's Gun aspect - grabbing a heretofore unmentioned pocket tool from its pouch/pocket is not as much of an Ass Pull
as many other things would be. Most people just would say, "OK, (s)he's got a pocket tool" and get on with the story - there wouldn't be a great likelihood of a disbelieving "oh, so (s)he just happens to carry one around" as it's a genuinely useful tool and the sort of thing someone who is out and about a lot might realistically have. Weapons and bricks on the other hand would need some serious explaining and setting up in advance - like what sort of adventures the character has that makes a weapon a good thing to carry at all times.
edited 27th Sep '11 12:19:08 AM by Wolf1066