Creating a blinking circuit (sensor probe)
If I understand the question, you are looking for a variable frequency and variable duty cycle 12 V timer but without using an electronic circuit. Since you're looking for a electrical solution it's probably going to have to be an electro-mechanical one. 

One suggestion is to use some 12 V car parts to make a cam operated timer switch. Window wiper motors, electric windows and seat adjusters have low-geared motors and some may have suitable speed control - wipers, for example, with control relays. All of these parts are vibration resistant which would be a bonus while 'in the field'.

For variable pulse rate you could try finding a rheostat to suit or maybe a car heater fan controller. Use this to vary the speed of the motor.

For variable duty cycle (% on time) you might need to make adjustable cams.

For switching, a roller switch on the cam should suffice.