Hi DennisR,
Look at my post of June 2018 (post no 7) which gives guidance from MX5 Parts as to how to wire up the side markers and kits of parts that they can provide. It may be that MX5 Parts now have a complete “illumination kit” for wiring up the side markers as suggested in their guidance or can give you guidance on why wiring to the brown wire at the front does not appear to work.
Hope this helps.
Cheers.