1992 Other Isuzu Models o2 sensor

This is an Isuzu pickup, the strippie model with the larger 4-cyl. engine. The manual says that the oxygen sensor needs to be replaced at 90,000 miles. At 90,000 miles, within a day or two, the O2 sensor light illuminated, and has stayed so ever since. It felt like it was programmed into the circuit to come on at 90,000 miles. Now, I have to go smog the truck, and the people who do tuneups on it say that it's going to cost me over $300 so they can completely take out the dashboard to get to the switch that turns off the O2 light. If the light stays on, I'll flunk the smog test. What can I do? Isn't there a backdoor way to kill the light? This seems like a really serious engineering snafu, if there's no way around it.