Lights dim and the speedometer varies going down the road. Also the cruise control and cooling fan quit working . Is this a problem with the wireing harness or a relay? Any help would be appreciated.This is a 1995 buick lesabre

Speedometer
The instrument panel cluster (IPC) displays the vehicle speed based on the information from the body control module (BCM). The engine control module (ECM) calculates the vehicle speed based on the controller area network (CAN) serial data message indicating the wheel reference speed from the electronic brake control module (EBCM). The ECM sends a CAN serial data message to the BCM indicating the vehicle speed. The BCM sends a class 2 message to the IPC indicating the vehicle speed.
The speedometer defaults to 0 km/h (0 mph) when any of the following occur:
• The EBCM detects a malfunction in the vehicle speed sensor circuit.
• The ECM detects a loss of CAN communications with the EBCM.
• The BCM detects a loss of CAN communications with the ECM.
• The IPC detects a loss of class 2 communications with the BCM.

Having the vehicle scanned for codes would probably point you in a correct diagnostic direction .

Blown fuse or relay

First I'll assume you haven't read the owners Manuel, Cruise control is not perfect on any vehicle. if coming to a hill put foot on gas to help rpm, going down hill put foot on brake to keep same speed, cruise control operates off of engine vacuum. Speed sensors can be affected by putting on smaller or bigger tires than what is on drivers door.

scan for codes first. sounds like a shorting wire. has vehicle been lifted?

Sure you can if a mechanic can do it so can you if you have the right tools and knowlege. First you need to know which one thers's one at each whell at the fround rear end housingf the rear rear end housing and the one in the side of the transfer case where the speedometer cable would'of connected on an older model and then theors three in the sides of the trans mission, Touque converter speed forward turbine speed output turbine speed. What proble are you trying to fix? the speedometer not working? Abs light on? Cheack engine light on? Cruise control not working? If the cruise control does work the speed senser is good and the check engine light will be on if the speed senser is'nt working. If the speed senser is'nt working the car will only take off in low and not shift to seecond or any higher gear as the computer doesn't know how fast the car is going. If the speedometer is'nt working and it does shift and cruise and the check engine light is'nt on then the speed senser is working but the speedometer is'nt if it is'nt and everything else is woring then as your driving hit the speedometer with your fist and see if it comes back on they can get stuck and i've fixed quite a few and they stayed fixed. On to wheel drive models with the rear wheels jacked off the ground safty I've run the car up to speed and trying wiggleing the wires and connectors till I wiggled the right thing till the speedometer came on ( including smacking the speedometer ) Just don't hit it too hard as it'll bust just like anything else.

find a set of wires at the rear of the transmission it should be the speed sensor wires make sure that they didint come loose.

It is most likely the speed sensor in the transmission.

Pretty common problem with these. Here's a guide to learn how to fix GM instrument clusters yourself, I used it to fix mine and it worked great.How to guide for Impala instrument cluster repair

Its your ABS system. There is a faulty switch/sensor on the master cylinder. It leaks brake fluid into the wires connected to it and causes the circuit to short, resulting in a blown fuse that controls the AC, odometer, speedometer, cruise control and ABS light to stay on. This can even cause a fire in the vehicle. A fire can occur even when the vehicle is turned off. To correct the problem go to this link - http://www.fordwindstarrecall.info/

try replacing vehicle speed sensors. you need a good vehicle speed signal inorder for cruise and tha speedometer to operate properly