As far as remember, the radiator side has black earth wires which go into the wiring harness/loom, and earth through the L bracket which holds the base of the wooden dashboard, which is bolted into the metal chassis. So all of that side go to -ve via the ring connection attached to the L bracket bolt.
The lights on the other side have black wires that connect to a ring connection that is bolted to one of the 3 bolts which hold the chassis closer plate at the front, to the underside of the fibreglass body. So that then earths through the chassis closer plate (which rusts badly being exposed directly to a wet road), and then onto the chassis via the 3 x 1/4" bolts which hold the rear of the chassis closer plate to the chassis main front box section.
I ran another thick black wire from the ring on the bolt on the front chassis closer plate through to join the ring connection to the L bracket as I thought an internal earth connection to the chassis would not rust up and lose connection. Lotus -ve earthing is notoriously unreliable, presumably down to corrosion at the connections, so I thought I would try to reduce that problem with the front lights wiring.
The new black earth wire is quite easy to thread through where your feet go, from the front ring connector to the L bracket connection to the chassis at the base of the wooden dash, about where the ashtray goes, if you are lucky enough to have one.
I have a large collection of ashtrays undergoing cleaning and restoration. I will post a picture soon of them all together so you can all drool over them with great envy.