I am replacing my battery in my 1990 F-150 and I'm wondering if the previous battery was installed backwards, because it is seems to be installed upside down. Does the positive or negative terminal need to be connected to the solenoid on the fender?

Sorry for the dumb question, I've been searching for an answer in this forum for 30 minutes. Thanks.

 

Not sure of your question, but the positive battery cable connects to the battery side of the starter solonoid on your fender (front of car). The starter side (firewall side) of the solonoid should have a large cable that connects to the starter motor.

Batteries do come in different configurations i.e which side the positive and negitive terminals are located, as well as amperage, storage etc.

If i understand your question it is possible that your old battery was not the exact/correct model for your truck. Either that or your new one is wrong.
As long as your cables can reach it does not really matter, as long as you have the correct CCA, or cold cranking amps to drive your starter.

 

Positive to the solenoid.