Taillights blinking (slightly) alongside blinkers. Bad ground connection?

[Kalimsnor]

Hi! When turning on my turn signals, the tail lights also blink alongside (only very slightly). Is this caused by a bad ground connection?

[Brianmoooore]

Yes, but it may be internal to the bulb carrier assembly (depending on which of the three versions you have) or at the plug and socket where the bulb carrier plugs into the car, rather than at the earthing point in the car (LH side of rear seat).
Do the front parking lights light slightly as well?

[Kalimsnor]

Yes, but it may be internal to the bulb carrier assembly (depending on which of the three versions you have) or at the plug and socket where the bulb carrier plugs into the car, rather than at the earthing point in the car (LH side of rear seat).
Do the front parking lights light slightly as well?

I have type 1 lights:

The front (parking) lights don't change at all while blinkers are on, just checked.

[Brianmoooore]

I was assuming post facelift lights, but most of what I wrote still applies, although I don't know how many different versions of the carriers were made, or their relative reliability/

[Ukhozi]

Definitely earthing problem, check out spring tabs on side of bulb holders, only takes a small amount of corrosion to stop it working.
I had similar problem when I bought a UK right holder so I get two fog and two reverse lights, quite grotty connections..