What's the name of this game?

Hi, I remember playing this game when I was a kid (around 2003, 2004) but I have no idea what it is! I'll tell you some details I remember but the memories are vague. It was a sci fi RTS/FPS game where you could enter vehicles and kill aliens but also exit and shoot them in a FPS style view. It was 3D with graphics that looked a little worse then Halo CE and I think it was released around the year 2000 and the game was always set on another planet with weird colours. It was on PC and you could also kind of get an overhead view and place defence towers that would shoot enemies, though the enemies were always in vehicles, I don't remember them being outside of them. I distinctly remember a little part of the game where an enemy was in a land vehicle and I had to shoot a tiny window on its side to kill the alien inside, I'm pretty sure the vehicle was black and yellow coloured but I don't know... I also want to stress that it was mostly set in a forst person view, so it wasn't anything like AoE or anything... I realise this is very vague but it was a long time ago and I remember really enjoying it... If you could hint to games you think sound similar to it that would be great, thanks for your time :3