1995 f350 speedometer stopped working, odometer shows no numbers and overdrive light is blinking with rough shifting. What can the problem be?

Your best bet , take it to a qualified repair shop. The overdrive off light flashes is telling you that the transmission has a problem an code's are set in the transmission control module or PCM - engine computer . Speedometer an odometer not working could mean speed sensor is bad . Checking code's with a factory or professional scan tool is need . The heater fan not turning off is a seperate problem , relay stuck closed maybe . Is it running at high speed ?

If the speedo and odometer are both dead, you should have shifting problems and a check engine light. The speed sensor in the transmission has probably died.
If the speedo does not work but the odometer does, chances are the problem is in the speedometer head.

Vehicle speed sensor. Top of the tranny is a connection. From this, a cable runs to the back of the odometer. Probably need to replace it

Usually it means the speed sensor in the transmission is failing.
The blinking light means your overdrive may not work.

Sounds like two different problems - the brake light switch and the speed sensor in the transmission.
Does the speedometer work ?

if the speedo is not sending a speed the trans will go into back up mode and the trans will not shift normally check the speed sensor

how to stop over drive light blinking and shifting hard

It sound is if your ABS/Speed sensor is the culpret.  Try this fairly inexpensive repair before you bring it in for a Diagnostic, which can run anywhere from $89.95 & up.  Go to the local ford dealer in your area and ask them for the ABS Sensor in your rear axle differential. The base part number being 9E731. This part runs roughly for about $20.00+ and your own time to replace it. 

there is a speed sensor on the top of the rear differential. either the sensor is bad or it has become unplugged. when the truck doesnt know how fast it is going, the trans goes into "limp mode" to protect itself...whic causes it to shift hard and if u look you'll notice the o/d light is blinking

OVERDRIVE ENGAGEMENT IS TIED TO THE SPEED SENSOR SIGNAL, THE SYSTEM WILL NOT GO INTO OD UNTIL THE CAR REACHES A CERTAIN SPEED, IN MOST CASES AROUND 35-40 MPH, THE SPEED SENSOR SIGNAL TELLS THE ECU WHEN THE CORRECT SPEED HAS BEEN REACHED.