Finally found out a way - it's quite easy to do, and it works on
every mission/mod. Survivors, bandits, wrecks, vehicles, tents -
everything will now spawn, regardless if you're on foot or in a vehicle!
1. Decompress your mission.pbo file with PBO manager.
2. Open the mission.sqm with notepad or any basic text editor.
3. Replace every "player in thislist" by "(vehicle player) in thislist" using the replace command of your text editor.
4. Save, repack your mission into a new pbo using pbo manager and put it in your mission folder.
5. You're good to go! Enjoy!
...and it only takes a few seconds to do.
NOTE : You can also skip 1. and 4.; just open your mission file with pbo manager, cut/paste the mission.sqm file on your desktop (make a backup copy, just in case). Edit it, save and put it back in your pbo folder. You're done! :D
EDIT : I don't recommand editing the "survivors" triggers on older releases. Unless you wanna have dozens of them in no time :P