I think the subject has been covered, but there is one point I want to clarify...
It isn't that your alternator is putting out less amperage, it's that it's taking more current to blast through the crud...
What was a nice clean circuit that only took 'X' watts to power when new is now taking 'X' watts to power the device, and 'Y' or 'Z' watts now because you are powering all of the resistance and corrosion too.
You are loosing so much to the resistance from the corrosion, rust, gunk, moisture, ect... it takes twice as much current to power the device...
It's the same alternator you started with, but now your demand on it has increased by 1/3 or 1/2 because of all the losses to the support systems (wires, connectors, switches, ect.).
Why do you think I stand on my soap box every day and extol the virtues of Cleaning Connectors every time you take them apart and using Dielectric Grease in them?
Cleaning them makes a low resistance contact, and the Dielectric Grease keeps moisture and oxygen out of the connection...
How about Rosin Core (NEVER ACID CORE), Silver Bearing Solder for connections?
Silver doesn't corrode internally. Silver is the best conductor of electricity. Silver is more dependable than tin or lead. Silver self heals when the connection cools, and can be remelted if necessary with out damage.
Silver content will keep the dissimilar metals corrosion out of the joint.
How about Heat Shrink Tubing?
Heat Shrink isn't cheap by any means, but it's a great way to seal the moisture and oxygen out of a connection.
It's more trustworthy than 'vinyl electrical tape', it's easy to use, damn near fool proof, available everywhere, seals up connections better than anything I can think of , and looks good to boot!
How about Crimp Connectors...?
Crimp connectors are a blessing and a curse.
Crimp connectors save you the trouble of using solder to Tin the ends of wires, and keeps you from having to solder and de-solder connections all of the time, but they have their own problems...
Dissimilar metals will corrode when in contact with each other... If the terminal isn't say, made of copper like the wire you are attaching it too, (and most cheap ones aren't), you will have a built in corrosion factor as the dissimilar metals react to each other.
You can stop this with...
1. Solder. Silver bearing electrical solder (not silver solder) will stop the problem before it gets started.
2. Quality Connections. Use Copper connections when ever possible. Lots of connectors will have a tin, silver, or cadmium plating on them, but if you look closely at the edges where they were stamped out, you can still see the copper (like a quarter).
3. Oxidation guard. Like dielectric grease or RTV in a pinch. Squirt some in the connector before you stick the wire in and crimp it. This will keep the problems to a minimum, and delay the reactions for quite some time.
4. Don't use cheap wire. Use virgin copper wire, not alloy.
5. NEVER PIERCE THE INSULATION OF A WIRE!
I've seen those 'Poke-A-Hole' test lights, and they are a really BAD idea...
Use a wire, jumper, probe, ect at the terminal, but NEVER PIERCE THE INSULATION OF A WIRE!
6. Know the proper way to make a connection.
A. Crimp, Twist, Ect.
This is a MECHANICAL CONNECTION, not an Electrical connection.
You may have low resistance now, but with a bare, air gap filled crimp or twist it's not going to be long before moisture, oxygen and the electrical current have their way with the connection and corrosion starts in a big way...
B. Solder, this makes the electrical connection, and should be done after the crimp.
This doesn't make the connection 100%, but it will be a lifetime connection if you do the next step...
C. Protect. Use Heat Shrink, Tape, Liquid Sealant, Ect. to protect the joint.
Moisture, Oxygen and Electrical Current are the enemies, and you can't do a damn thing about the Current flow...
Mechanical Connection, Electrical Connection, Protection....