I can't find the Pontiac Sedan Six. This car DOES NOT exist in the game. Why must there always be the few cars that just do not spawn at all? 
 
If anyone knows of anywhere to find one PLEASE tell me. I have 90/95 and I might know of some if you're having trouble too.

The guide off of 360a I used stated this:
 
18. Pontiac Sedan Six
Location: Downtown, Central. This can be found ambiently Downtown.
 
Credit given to Grifftuitous.

I know how shitty it is finding all those cars (I got the trophy earlier today), so I'll try to be helpful. 
From L.A.N. wiki: 
Recent reports suggest finding this vehicle in Downtown, Central. But it can also be found in Hollywood near the Polar Bear Ice Company. 
You should also take a good look at this video so you know exactly what your looking for: Pontiac Sedan Six.

Sigh... and here I thought GTA's pigeons and AC2's feathers were bad....

Sigh... and here I thought GTA's pigeons and AC2's feathers were bad....

Try going for all the flags in Assassin's Creed 1. There are over 300 of them.

That said, I still haven't found a single bloody newsreel after completing one playthrough, plus various case-replays.

@xyzygy said:

Try going for all the flags in Assassin's Creed 1. There are over 300 of them.

 
Forgot about those, only ever bothered collecting the first set in Masyaf, and gave up. If there were only one type of flag, it may have been bearable. That said, I still haven't found a single bloody newsreel after completing one playthrough, plus various case-replays.

It's a fucking annoying task to find all these cars because they're all randomly generated. Every time I go to a location that a guide told me to go to the car is never there. And now the game says I have 91/95 even though I had 93/95 before.

My advice is to copy all the car names into a single word file and erase all those you've found (all the cars are listed under main menu, extras). I got Platinum a second ago, and I followed these guides. Good luck!
General 
Cars 
Golden Film Reels