You can't gain access to the dashbulbs on a Xantia from under the footwell, you have to pull the panel out of the dash which on a S1 also involves removing the steering wheel, TBH its not that much extra work.
I take it that as your Xantia is a 95 its not fitted with A/C? If the heater slider isn't working its more than likely due to a cable fault such as a missing clip, remove the radio and look down onto the heater panel to see whats happening when you move the slider. To look at the other end of the cable you might be able to view it removing the glovebox (don't forget there's two screws in each corner under the felt).
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