I was given the task by my boss to troubleshoot his "big Problem". It's a 2005 Fiat Ducato, diesel with manual transmission.
Problem: The front left park light (clear) and right rear park lights stay lit after the key is removed and all switches turned off. These two lamps stay lit and will not turn off. All other bulbs function as normal. This van is equipped with with fog lamps on both rear lights. They're the only bulb that has a double filament. I have already checked and cleaned corrosion. Disconnected the rear wiring harness to see if the front lamp is a related problem with no results. Has anyone else seen this problem before? If I'm a betting man I would guess it has a bad ignition switch but I can't guess because this is a customer's vehicle. Does anyone has a location for the wiring download?

Hi, Welcome to the FF.

Check the forum downloads section here: https://www.fiatforum.com/downloads.php?categoryid=22

Top of the list is a Ducato wiring diagram written in Spanish. I don't know what year Ducato it covers.

Further down the list are 2 different eLEARN manuals. These are full workshop manuals that include wiring diagrams.
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Interesting problem; if I were you I'd start by removing the relevant fuses for these lights. This will break the problem down to before or after the fusebox.

(If after this would eliminate your concerns about the ignition switch although I do recollect that parking lights are switched on by means of this switch)

I had a wireless rear view camera fitted to my 2005 2.3 diesel MH and I had lots of problems with both side & marker lights staying on all the time.I disconnected the wireless sender and hard wired the camera and since then no problems at all with lights.
This may be totally irrelevant but I thought it may help.
Best of luck
Nigel1947

Faulty ignition switch. First position with the black button is to keep the parking lights on when the key is withdrawn.