Only one speaker works

a bit vague ?
I am assuming you are trying to connect a sub wooofer and a normal speaker of some kind. A sub woofer works for low frequencies so you may need a crossover to pass low frequencies to the subwoofer and other frequencies to the normal speakers.
The other reason may be the impedance of the two speakers being wired together may make the total impedance out of the normal impedance level of the amplifier.

When the System comes back on in Safe Mode, all unnecessary utilities are shut down or do not work, such as your speakers or the sound. If you open Safe Mode with Networking, than you are connected to a Network, but you still can only do a few tasks that Safe Mode allows. After you restart your computer and run it normally, than all functions will return to their normal operations, including your speakers and sound.

The power amplifier is not working. Find a repair shop near you to find the repair rates, then decide if it's worth fixing.

THE DOOR SPEAKERS ARE MADE TO SOUND LOW FREQUENCY THROUGH THE AMP,(60HZ-500HZ). THIS IS NORMAL. THE REST OF THE SOUND WAVES (500HZ -8000HZ) COME FROM YOUR 2 TOP SPEAKERS ON DASH NEAR WINDSHIELD, 2" TWEETERS.

CHANGE THE SPEAKER

Sound issues are existent withinin vista onwards as is decreased from normal as with xp and previous. To fix problem of this depends on installing an appropriate os system...

Have you tried another speaker? If your left front speaker is in the door as in most cars, it gets banged around a lot. The driver's door gets the most use. This sometimes results in mechanical problems with the speaker, and can cause low sound or distortion. Take the speaker cover off (it may be necessary to remove the door panel in a factory installation), disconnect the speaker and hook another in its place. If the volume is back to normal you know it's the speaker. If not, check the radio and speaker wiring. If the radio itself is at fault because the amplifier is bad, it can be repaired at a service shop. The repair charges might be close to replacement cost these days, so you might consider a new radio if the estimate is high.

you cant get echo on a specker there not made for that just normal sound

If all your connections are sound, call dell. If they can't do anything for you insist on a refund. (You have to get testy with Dell)

Hi Paulus
A lot brand of speakers, like Sound Blasters, Logitech, Cambridge, has this (strange) kind of volume problem, sometimes, is the resistor, in the volume control, sometimes in the modulator for the board on the sound speakers, Subwofer.
I use a general tip is let the volume in one setting and don't move in the intermediate low volume say....9 .
from other thing, be change the speakers.