When I purchased my '94 it has an aftermarket Kenwood Single DIN in the dash. It was a POS unit, but it did have a nice 140 Watt amp under the passenger seat. I kept the amp and replaced the Stereo with a Jensen DVD/Nav touch screen receiver. It works great.
On the SYS error on the instrument cluster, the computer is checking for bad bulbs in the radio head. When you install an aftermarket receiver the circuit between the radio and the computer is open. You can fake the computer by installing two 1k ohm resistors on the connector that went to the original radio head (dash unit)
This info came from a posting on Corvette Central:
1. Buy (2) 1 kilo-ohm 1/2 watt resistors. [Radio Shack part # 271-023. They sell them as a 2pk for .29 cents.] Note, 1/4 watt work too, that is what I used -- Bob
2. Disconnect the negative battery cable.
3. Remove the receiver.
4. Connect one end of both resistors to ignition power. (Yellow wire in the car)
5. Connect the other end of one resistor to the purple/white wire in the radio head connector.
6. Connect the other end of the second resistor to the grey/black wire in the radio head connector.
7. Cover all connections and completely cover the resistors.
8. Reconnect the negative battery cable.
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Bob - '94 Polo Green Coupe