[Devil]

I would allow people to roll knowledge skill checks in some situations. maybe give them a bonus dice for their active skill roll for each dice they get over the threshold of the knowledge skill roll.

I don't really like it when things use 1/2 of something. Since shadowrun rounds up that means that you should just stay on the odd numbers.

[Kontact]

If relevent, i allow the player to roll a knowledge skill test and every two hits reduces a modifier by one, but this only applies to tests where the player can take time and think, IE an extended test or a test that isn't done under duress. Repairing that Mercury Comet isn't gonna be any easier if you're in a gunfight, no matter how many ranks in engineering you have.

If you manage to survive in a gunfight for the 3-6 hours it takes for that first roll on a build/repair vehicle extended test, I'd have to tip my hat and ask just how many bullets your buddies brought. 

[Jeeves]

I've had runners pull off a situation similar to that. Their main transport, a slightly armored car, was in need of repair, and the runners had to hold off against a small army of goons that was chasing them out of pullyup. The poor rigger was doing all he could while the four other guys were using their guns like hoses, just spraying and hoping to soak something in hot lead. Luckily, ammo was plentiful.