Hi, I too have a Mk3, it's a 2001 ghia x manual 2.5 v6 and am having a problem with a flashing light. Your fault code is 51 find the fault here https://www.fordwiki.co.uk/index.php/Airbag_Flash_Codes - it only gives you 3 "What it might be" - it seems to be your drivers side safety belt buckle switch - do you have a little electrical knowledge and reasonable DIY skills? - if so you could probably do it yourself, but beware - trying to find a short circuit to ground is like trying to find a needle in a haystack. I've had resort to going to a (recommended) garage to get a fault print out from their pro "OBD2" software. It's costing me £49.90 coz I can't find my fault code 25 problem; but I can then replace what the problem is myself. I was told that a common mondeo problem is the Airbag ECU - you can get one on that well known online auction bay (don't know if I'm allowed to say the name) - for upwards of £12 which seems a pretty good deal to me. Sorry about the novel - best of luck with your fault finding.