TRACKMKIII

(1994 GS300) Hey all!! I installed some new speakers a few days and they sound great but today my stock sub decided to finally quit after 20 years. I found a good open air sub on Crutchfield.com for about $90 my question is how can I use it with the stock head unit. I saw a post about using a "HI/LO converter" and a small amp just to power the sub but the description was not very clear. If I can just wire it in using the stock amp that would be cool too. My other question has to do with the front speakers I installed. Once the speakers were installed I lost to the 2 extra prongs that send power to the tweeters so I just spliced in the the wires to power the tweeters in do so I apparently lost the built in crossover in the factory speakers so the tweeters sound horrible now. I guessing my best bet to fix this issue would be to use 2 passive crossovers and a set of new tweeters? Any recommendations for passive crossovers and the sub install would be greatly appreciated!!