ok guys - all i can say is that it takes patience - so do not give up, they are well worth the wait and you get better at getting them. its difficult to explain in more detail than before but ill try. by your portland hideout is a main road. on the side nearest your hideout (northbound traffic) line up some tanks (use the tank cheat or they will disappear when you're starting the mission). so that they are at an angle which points into the entrance (about 60 degrees seems to work) so that when the cars hit them, they bounce into the hideout entrance and get stuck in there.
that bit is simple if you understand what i am saying, but the next bit is hard to get the hang of. when youre waiting for the cars, sit in the end tank (furthest from the wall) and select the overhead view (if you try without you will understand why). follow the cheetahs in and blow one up. now comes the hard bit. you have to push the car (by tank appears easiest, although you could park a car in the hideout area and change once its blown up) to your garage door. wait for one of the other cars to finish the race (one MUST finish or it will not work) and the mission failed sign will come up. now push the car into the garage (make sure it is empty before starting this obviously) and the garage door will close (sometimes it will close on the car before it is fully pushed in - this cannot be avoided and it is bloody annoying but keep at it) eventually you will get it in and the door will close behind it. the next time the door opens there is a shiny new bp cheetah waiting for you. (these cars can be resprayed like any other car and not lose their bp strength - i have an army or dark blue ones (6) in my staunton garage and in the process of getting light blue ones for shoreside.)
HAVE FUN!