Well, the left front speaker and tweeter of my 98 GS300 just stopped working.
What could be the reason, where should I look 1st?

 

If both the midwoofer and tweeter are out then there' s probably no signal going to them. Maybe try to disconnect them and try another speaker to see if its not a speaker problem. Could be a wiring issue but very unlikely.

 

So, just open up the 2 front doors and swap.

Couldn't it be the headunit, because it is both the mid and tweeter?

 

Well if you have any car speakers laying around you can hook those up to test.

 

If both mid and tweeter are completely out, it's most likely wiring/connectors, but could also be headunit. The mid and tweeter would have never died at the same time. If you have stock audio, check the connector that goes into the plastic door enclosure behind the door panel, maybe it came loose.

 

I do not quite understand what panel you are talking about.
I'll try and get the headunit out in the morning, I hope one of the connectors fell out or something.
Driving without a good stereo sucks!!

 

The plastic enclosure for the midrange speaker is located behind the inside door panel. I'm talking about the WHOLE panel, power window switches, door handle and all. The whole panel needs to be removed to get to the speakers.

It's not likely to be your headunit connector - I would not start there. Take your door apart and see what's going on in there. Wiggle the connector that goes into the stock speaker enclosure while you have the audio playing.

 

OK, I got it now.
Sounds like a hell of a lot of work, LOL.