Put the battery in backwards. Now the car will not start. I know I blew the starter relay because the lights and radio didn't work inside. I replaced the relay. The inside lights work now. But the car still will not start... What else can I do?

backward is super huge mistake blowing up electrons on the car.
check fuses
crank and scan the PCm.

if lucky you only blew up the aternator./
its regulator blown up to H3LL. or pcm is the reg and blew it up.and very costly error.
this is not 1960 and no computers and cars could survive this error.
those days are gone. (and loved)

doing that (reverse battery,) blows up very extensive electronics.
fuse that look good can be in fact bad.
A pro never guesses, we use a magic $8 tool (walfart) called volt meter to find them.
every time. no guessing. the the mains are huge
too bad you never told what exactly what you did.,
1: removed battery plus cable first (wrong) and wrench hit steel>?
2: installed the battery backwards, boom"
3: removed the alternator , hot or starter (boom)

know what you did ,tells me which fuse blew., for sure.
the voltage will be on both sides of fuse. if fuses is good,some
need key on to be hot.
(rare exceptions)
some cars have fuse links that look like wire are are NOT.

you have no power to the radio ,now, the voltmeter is 0v
there.
so you back up from there
but why
why not start at the travesty area, the battery and check all PDC fuses first for 2 x hot each. need a ASE TRAINING video have it.
fuse that look bad are
fuses that look good,can in FACT be bad, gawd made is a meter
and bingo we are all blessed with truth.

Check all other fuses. Before the fuse blew, you can have damaged lots of electronics. Even almost 20 year old cars do have electronics and relays.

Dirty battery cables or faulty starter. Do the lights stay on bright while starting?

You need to take the Alternator in for a Bench check. The diodes are probably bad. Wiper switch or motor can be bad. Radio can be smoked.

For wiper motor, the contact points use a system where a derailer mechanically shifts the points to break contact. The contacts are probably fused.

When the battery was connected in reverse some electrical components were damaged. A motor, a relay, headlights can work both ways but not printed circuits in electronics. You are lucky you did not toast your computer!!!!!!!

With a voltage meter see how many volts are at the starter terminal. If not 12+ volts you have a bad connection. Check connection at the starter solenoid. Does it have 12+volts? Short the solenoid. Does it put voltage to the starter?

Check under the surge tank near the battery or in a box with all other relays.

It sounds like you blew fuses other than just the main fuse. Hooking the battery up wrong can destroy major components as well, like the starter, battery, Computer, moduals. You might need a good shop....

1. Loose battery or earth connections on bodywork/engine.

2. Starter stuck in mesh. (rock car backwards and forwards in gear to free)

3. Solenoid stuck. Tap lightly with a hammer.

4. Starter relay in or near fuse box faulty. Replace relay.