MKIII warning lights

Don't know the details of the RGM lamps but this is the type of terminal I use for 6mm and 3mm flat 'spade' connections (Lucar as they used to be known):

https://www.ebay.co.uk/itm/193115166878

(Other sellers are available.)

Try the local car/auto shop, they usually are sold for speaker connections. Beware, they will most likely be your favorite 'red' or 'blue' style crimp!

Or Auto electrical supplies has them as uninsulated ones and the matching sleeves   2.8mm female blade No 030201 £1.78 per pack of 25

I've been able to use the 2.8mm, with a bit of a heavy hand, on 3mm relay terminals.

But how about this:

https://uk.farnell.com/amp-te-connectivity/63705-1/female-disconnect-3-18mm-22-18awg/dp/2847080

or

https://uk.farnell.com/amp-te-connectivity/7-520366-2/terminal-female-disconnect-3-18mm/dp/2396272

Farnell are good for industrial connectors and suprisingly easy to deal with for small quantities.