covid 19 – Do I Need an FCS (Passenger Locator Form) if Entering Spain by Road?

It seems that you do not need it. From the consular office at Lyon

1.B. Entradas por vía terrestre

Todas las personas de 12 años o más, procedentes de países o zonas de riesgo que lleguen a España por vía terrestre deberán disponer de alguna de las certificaciones que arriba se recogen (de vacunación o prueba diagnóstica o recuperación). Quedan exceptuados de este requisito:

That basically says that everybody over 12 crossing into Spain through land must have a valid CCD (minus a few exception for transportist, people living near the border and people who must cross the border for work regularly).

Also, point 2 is:

  1. Formulario de control sanitario (entradas por vía aérea o marítima)​

which means “Sanitary Control Form (entry by air or sea)” which makes it rather clear that it does not appy to land travel.

A bit of a worrying point is that it tells that people

tiene que llevar consigo un certificado digital COVID emitido por un estado miembro de la Unión Europea (UE)*, que certifique alguno de los siguientes requisitos sanitarios:

must carry a COVID Digital Certificate issued by a member of the EU, which the UK isn’t, but it looks more like an oversight than a refusal to accept UK’s certificates.