You want to do WHAT with your negative cable disconnected???? You said tha you disconnected your battery from the firewall??? I´m assuming this was a small wire about 10-8 guage or so, and that the actual cable itself is still connected to your engine block. Is this correct? The worst thing that could happen if you removed the small wire and left the big wire is some of your body ground points might have a little more resistance than they used to, but if you removed the big wire and left the small wire then I don´t forsee your engine starting any easier (if at all). As far as the putting a new terminlal on the and of the old battery cable, that isn´t really a good fix at all. I would sugest replacing the whole cable. There could be more corrosion under the insulation that you can´t see. And as fas as getting rid of corrosion on a battery cable, the best thing in the world is not at a parts store, but in your kitchen. Mix a table spoon of baking soda with a cup of water and mix them thouroughly. Then poor it on the corrosion and watch it sizzle away. then wash off with fresh water and viola. And if you are ever in a pintch, you can use Cocoa-Cola. Scary thought huh? We drink it everyday, but yet it can eat away batterty acid. Go figure. Also another way to avoid corrosion is to make sure you cables are always tight, and clean your battery regularly.