Ford F-150 charging system

1987 ford f-150, just an old beater truck nothing fancy! I recently purchased this truck from a friend. Just so ya know, the rod that engages the ignition switch had busted so the column was tore apart and now the switch is hanging from the column. Every once and while it doesn't want to start and by bending the switch back and forth, which bends the wiring, it takes off. But the real problem is the battery draining. With a full battery the truck starts fine and then not too many miles down the road will suddenly die. I did notice though that if I get it up over 3,000 rpms the lights brighten and the voltage on the instrument cluster jumps to around 12. Once it dips below 9 or so it seems to be the point of no return. Well I just put a brand new $90 alternator and it appeared to run fine the last two days puttin' around town. The 3,000 rpm thing doesn't seem to take effect either. Although the voltage guage, if correct, holds steady at about 9. But now the kicker. Today it took out on the highway 15 miles south and on the way home, even after letting it warm up for 20 minutes or so, I got maybe 6 or 7 miles down the road and thought the dash and headlights looked dim. Another 3 miles and the voltage was at 8. A little further and it was just under 8. Usually at this point it just dies but a few more miles and was home and even in the driveway it came close but wouldn't die even while sitting for a few. I should also note that it appears my wiper motor is shorting out. But even so, the battery should stay charged down the road! Help i'm cursed with vehicles that short out batteries! What should I do? Oh yeah I also had replaced the solenoid when I got it 'cause it wouldn't even start, and no my friend never had this problem when he had the pickup. Thanks guys!