You dont NEED an amp, but it will definetly power them to their full potential.
If you are using a stock head unit, I would highly suggest upgrading to an aftermarket head unit, as the newer models will have more power than an old 89 weak radio.
Just install them and see for yourself. If you want them to be a little louder, you can easily find a decent used older 4 channel amp for around 50-75 dollars.
LOL, definetly dont need another battery! Adding more batteries is for people with thousands of watts and upgraded high output alternator.