my 1998 ML320 ets light is on. Is this serious?

is there a check engine light also? there are still too many possibilities, as to the cause, if ther eis a check engine light, any chain parts store will check it for free. If a code is found will start with a :C and 4 numbers...any solenoid may have seized, connector may have corrosion, the brake switch, so lets start with indicator lights....

Probably a brake light switch.

yes, the brake light switch potentially can be the fault, the common find on the ml, is the left rear, solenoid but the check engine light info would help me more...

i just had the same problem my ETS light is on my cruze control dont work when a step on the brake paddle down to floor starting up the car notice ETS code not on i went down to auto store $20.00 dallor brake switch problems fix