The red wire goes to a black terminal and the black to a red on the horn. Is this wrong and could it be why it is not working

There are really only two choices. Either the horn amplifier in each cab is blown, or the high frequency horn driver diaphragm is blown. It's easiest to check the diaphragm first. Open up the cabinet so you can get to the horn driver terminals. Put a voltmeter across the terminals and check for continuity. if it shows open, you've found the problem. Open the driver and replace the diaphragm and you're done.. If the horn driver tests good, it's time to take the cabinet into your local electronics repair place to have them check out the amp section.

Horn is red +, black -, mid is white +, blue-, lows are yellow+, blue-. If you look at the crossover board, you'll see the treminals are labeled what goes to where.

In Peavey cabinets, the woofer is always Yellow + and Blue - and the horn is always Red + and Black -. On the mid the remaining two wires would be the lighter color + and the darker color -

In Peavey cabinets, the lower woofer is always Yellow + and Blue - and the horn is always Red + and Black -. On the middle 15" the remaining two wires would be White + and Blue -

See https://www.diyaudio.com/forums/multi-way/99647-magnavox-horn-specs.html

Peavey color indicates that Blue is Negative And Yellow Is Positive, so Yellow goes to + Speaker Terminal.

black and red will goto 1 speaker. and blue/yellow to the other. typically red and yellow are + and black and blue are -. the crossover board might tell you which is which where they solder to it. but i'm pretty sure the black/red goto woofer. try hooking up both to woofer and listen to it. 1 way is going to sound horribly wrong lol. that will mean you hooked up the tweeter leads to the woofer. but DO NOT HOOK UP WOOFER LEADS TO TWEETER. thats why i say to do it on woofer.

Usually the colors you will find are red and black pair and yellow and white... USUALLY the red/blk pair go to the tweeter horn with red going to the + lead of the horn drivers. Usually the yellow/white pair go to the woofer with the white going to the + lead of the woofer in Peavey speakers... HOWEVER you never know if someone has rewired the cabinet so going by colors is NOT the best way to go... This should have the "soundgard" circuit in it and the lead that goes throught the lamp and resistor that is NOT connected to the inout jack is the TWEETER + lead. The pair that connects to the inout jack DIRECTELY is the woofer leads and the TIP connection of a 1/4 inch jack is the + woofer lead. Verifying these connections is the best method.

Most schematics won't even have any colors listed... so let us have you try this:

Find a system ground someplace like the sleeve of a 1/4 inch jack... Using an ohmmeter check for continuity to the available wires that are disconnected. ASSUME that those showing a low resistance will go to the "-" terminals of the drivers. If you have say a black and red... expect that the black will be one of the grounds... often the red then will be the (+) of the woofer. If you have whites and yellow... OFten the white will also have continuity to ground and be the (-) for the tweeter and the yellow then to the (+). Now since your speaker has 4 woofers we need to be careful to wire then correctly to get the 4 ohm impedance of the cab. IF each woofer is 4 ohms impedance, then we need to wire them in series-parallel. The shell of input 1.4 inch jack will likely go to black wires... two of the speakers will have their (-) terminals connected to these. The tip of the 1/4 inch jack will probably connect to one or more red wires. These will connect to the (+) terminals of the OTHER TWO woofers. Next one of the available (-) with no connections will tie to the (+) terminal of one of the first two woofers. SImilarly the remaining (-) terminal of one of the woofers will tie to the last (+) terminal that is vacant. Note the + of the horn will tie probably to the High level pot and this will tie to the crossover someway.
Here is a site with general schematics.

http://www.bcae1.com/spkrmlti.htm

If this has active circuitry, one of the wires is the cutoff for the power supply from the battery. The other two are black for ground and either white or red for signal.