Actually, you have to declare at its current market value. You can obtain this value online looking on websites for purchasing cars.
However, there is a much simpler way. You can amortize your car at a constant annual rate of 40% (for Zurich) subtracting it from its remaining value.
A simple example. Purchase price CHF 28,000. Year of purchase 2018. The tax value for the tax return 2018 would be 28,000 * (1-40%) = CHF 16,800.
For the next year it would be then 16,800 * (1-40%) = CHF 10,080
So, you can see it’s pretty simple.
Hope it helps.