"Caps" or capacitors can be found at radio shack or other electronics store. They do make a considerable difference, however, in an older car, the "subs" or sub woofers will receive a greater amount of frequency blocked by the "caps". This can lead to blown "subs" very quickly. (it did in my case) but I am in a 10 year old car. Do yourself a favor, If your serious about your sound system, ( I thought I wasn't until I got started working on it), go ahead and get a decent head unit..... One that doesn't cut off for no reason driving down the road..... Get something with Aux input, and/or BT. Marine Radio's tend to be the same color as our trim. Door speakers are of excellent quality, but the subs are like unicorns. If you want to keep it simple, find a core set of Subs and have them re-foamed.

The stock amp can be wired to make a decent head unit sound a lot better!

BTW. I have stock subs that need to be re foamed. PM me if you want to be pro-active and have a set sent for repair so when your subs give up, due to time and carrying the extra load from the "caps". I'll let them go for a fair price and ship them to the repair station of your choosing.