Use [this page][1] to generate SPICE kernels for the bodies of interest, and then use [SPICE routines][2] to calculate whatever you like to your heart's content.

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Aarrr.  I don't be knowin why ye be reinventing th' wheel.

Alrighty ya scurvy bilge rat, here it be:

$$x=a\left(\cos\tau-e\right)$$
$$y=a\sqrt{1-e^2}\sin\tau$$
$$z=0$$

That be givin it t' ye in th' plane.  Then ya be rotatin th' plane with that land lubber Euler's transform in $\Omega$, $i$, $\omega$ (longitude of ascending node, inclination, and argument of periapsis), by multiplyin his matrix by yer vector up there:

$$\left(
\begin{array}{ccc}
 \cos\omega \cos\Omega-\cos i \sin\omega \sin\Omega & -\cos\Omega \sin\omega
  -\cos i \cos\omega \sin\Omega & \sin i \sin\Omega \\
 \cos i \cos\Omega \sin\omega+\cos\omega\sin\Omega & \cos i \cos\omega\cos
  \Omega-\sin\omega\sin\Omega & -\cos\Omega \sin i \\
 \sin i \sin\omega & \cos\omega \sin i & \cos i \\
\end{array}
\right)$$

Ye also be needin th' time it be ($\mu$ being th' $GM$ of the Sun):

$$t=\sqrt{a^3\over\mu}\left(\tau-e\sin\tau\right)$$

Yer $\tau=0$ and $t=0$ being yer time a periapsis.  Ye be seein that ev'ry $2\pi$ in $\tau$, it be one a yer spins about th' Sun.

If yer scurvy rock be gettin close to a planet, then ye be wastin yer time, as th' orbit be changin on ye.

  [1]: http://ssd.jpl.nasa.gov/x/spk.html
  [2]: http://naif.jpl.nasa.gov/naif/toolkit.html