Willi does actually say theres a bomb on the truck (If thats where you are stuck)
The bomb is on the truck right next to you and Willi, you need to know the correct order of wires to cut. HINT : flag
MOve your cursor over the truck and the 'I can use this' icon appears.
If thats not where you are stuck then escort the truck a bit further and some army guys intercept you , you can bribe them, or fight them. I suggest bribe because teh fight is next to impossible and you still make a lot of cash even if you bribe them.
If its even after that you are stuck then you must wait for the truck to arrive in cali cantos. But I recommend not leaving the truck you have to escort at all, stay with it on the world map.
Make sure you dont lose sight of the truck that you are to escort while on the world map and you will be OK. You can actually click on the truck that you need to escort in some way and you will follow it very closely, thats the best way to do that mission so that you dont miss any encounters.