ive got a late '91 940 GL Wagon. Its got a 4 speaker sound system and ive got some spare speakers. I read that the front doors are wired for speakers like the rear are for windows. I tried plugging in a speaker to the wire on the drivers door to no avail. Does my head unit/amp not support 6 speakers or do i have to modify something?

Which head unit do you have, and are you sure whether or not there is an amp attached or attachable? E.g., the CR-814 runs two off the head and four off the amp, but the CR-915 runs all six off the head, with no external amp. The head units of that vintage tend to run the dash speakers direct off the head, and any other speakers off an amp, but there are exceptions as noted.
If you have an amp, but for some reason it's not doing anything for your speakers (you can tell, if there's a skinny black cable with a round din plug connected to the "amp" slot on the back of your head unit), check the connectors befrore you pitch the amp. If there is an amp, see if it's a 3533004 part number( typical 4 x 20) or some earlier 2x whatever. If it's the 3533004, you can double the output by a direct plug-in of a 960 amp, part number 1373082.
If you go to doubling up speakers in parallel, you should go to 8-ohm speakers, or there may be a frying sound and a bit of smoke where your amp used to be when you crank it up--the single-speaker channels are for 4-ohm speakers, and if you wire two speakers in parallel, you need to go to 8 ohms because the electrical gods will be pissed otherwise. Don't ask me why two 8-ohm speakers are the same as one 4-ohm speaker from the perspective of your amp. There are fancy formulae for all that, and somebody really smart has already posted the necessary explanation.
Whatever else you do, get the wiring diagrams first, and draw out any changes you make so you'll remember what you did next time you want to make more changes.
Another option is to get a splitter made for the amp cable and run two amps, giving you six or eight 4-ohm channels so you can use ordinary speakers.
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We have met the enemy and they is us. [Pogo] '99 S70 cop car : Rough Rider tires& suspension, Walmart fog lights, speakers by ebay, ambiance by Pall Mall, trim by Le Duc d'Tape

doors are wired, amp will appear to be wired, but the amp is only 2 chanel (driving rear doors), dash speakers are driven from head unit...

solution 1:-

upgrade amp to 4 channel amp..

solution 2:-

throw head unti & speakers & amp in bin , fit new head unit & speakers..note you will near wirring harness adaptors for this..

ive had a brianwave. could i not just cut into the wires to the dash speakers and join them to the door speakers? that'd work wouldnt it? i could have both dash and front door speakers working off the same channel? would there be any noticable degradation?

Well-- the better solution is to get the bigger Amp, which fits perfectly. It was also used in the 740' where I got mine from. You won't need to do no rewiring at all.
If you want to paralell the speakers, then do it at the multiplug coming from the Amp at the back of the amp. I think you may run into a resitor problem with it--.

850's have 8 speakers which run off of the 4 amp outputs. The trick is to place 4-ohm resistors at EVERY speaker to make the load at each speaker 8 ohms, so that when two speakers are wired in parallel it creates a 4-ohm load. Neat trick that works very well.
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'94 854 N/A.

hmmm it may work, but not exactly recomended practice...mainly because speeakers dont have a static impedance.....

also taking the dash feeds into the full range speakers will sound sh*t...mainly because the head feeds to the dash speakers dont have a full bass end o nthem becasue the speakers cant deal with it...

do the job properly...or dont bother...