Before we go any further, this is the Leon forum so I assume we're talking about a Leon here and not the Ibiza2000 that your forum ID might otherwise lead me to assume?
Manual dials means that the cabin air blower speed is controlled by a resistor pack attached to the bottom of the blower itself. The blower is on the offside, deep behind the dashboard and under a cover. It's covered in both of the Haynes Golf/Bora manuals (3727 or 4169) that are good for almost all Leon I and Toledo II maintenance.
A bundle of wires goes from the fan speed dial to the resistor pack. The resistor pack is just three resistors in series, and the switch connects power from the fan fuse (25), through the resistors to the fan motor. Setting 4 (full speed) bypasses the resistor pack and connects the motor to power directly, hence it is a good bet that the resistor pack or the wiring or connectors to it are out of order.
You could start by metering the resistance between the wires that connect to the switch contacts.
ASCII-art follows:
Code:
Fuse 25
|
o Fan Speed Switch
|
/ / \ \
/ | | \
/ | | \
/ | | \
| | | |
|----R1---R2---R3-- Resistor Pack
|
|
Blower
The resistor pack has an overheating fuse built into it. If this has blown you need to look at the fan motor and see why it might be getting too hot.
The heater control dials are lit by a bulb or an LED mounted behind the fan speed knob. It's apparently quite difficult to extract, needing pliers padded with a cloth or something similar. If you have a bulb in there you can replace it but it is well buried and will need needle-nose pliers (and a bit of care) or a tubular bulb extractor